The 19th season of the Top Chess Engine Championship began on 6 August 2020 and ended on 16 October 2020. The season 19 superfinal was a rematch between Stockfish and Leela Chess Zero, the same two engines that had contested the superfinal in the previous two seasons. Stockfish, the defending champion, won by 9 games (+18 -9 =73).[1][2][3]
The format of TCEC Season 19 is largely unchanged from that of Season 18. The only difference is that the time control in the superfinal is once again 120 minutes + 10 seconds per move, increased from 90 min + 10s in Season 18.[4] The rules are also largely unchanged, with the only significant addition being r-mobility as a tiebreak criterion. This tiebreak depends on the number of legal moves available to both sides in the last phase of the game; the side with the fewer legal moves loses.[5]
Tiebreak rules
The tiebreak rules for TCEC Season 19 are:
For any event except the superfinal: the number of crashes.
New entrant SlowChess was the early favorite in the Qualification League because it had shown a great deal of improvement in self-play in spite of being a relatively new engine. It lived up to its billing by dominating the Qualification League, scoring 16 wins in 22 games, including head-to-head wins against all its opponents. Second place was much more fiercely contested, with Roc eventually squeezing past Combusken in spite of losing both head-to-head matches to its rival. Eventual fourth-place finisher Monolith was also in the running up until the last rounds, but it lost both its games against SlowChess as well as a game against Roc which eventually left it 1.5 points behind Roc.[1]
At the other end of the table, the other new entrant A0lite finished last together with Bagatur, scoring 5/22.[1]
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
SlowChess
22
16
6
0
19
Advance to League 3
2
Roc
22
11
8
3
15
3
Combusken
22
8
12
2
14
4
Monolith
22
8
11
3
13.5
5
chess22k
22
6
12
4
12
6
ChessFighter
22
4
15
3
11.5
7
FabChess
22
4
13
5
10.5
8
Weiss
22
5
10
7
10
9
Asymptote
22
2
14
6
9
10
Tucano
22
3
9
10
7.5
11
Bagatur
22
1
8
13
5
12
A0lite
22
1
8
13
5
Source:
League 3
Although the competition was much stronger, QL winner SlowChess also crushed League 3, scoring 13/18, 1.5 points ahead of second-place and third-place Igel & Minic. This time, however, there were three engines it was not able to beat. Comparatively the fight for second place was very close, with Igel barely squeezing past Minic on the strength of a head-to-head win. This result will later turn out to have a significant impact on League 2. The rest of the league was straightforward, with iCE and Counter finishing solidly at the bottom, losing 7 games each and 2.5 points from safety.[1]
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
SlowChess
18
8
10
0
13
Advance to League 2
2
Igel
18
5
13
0
11.5
3
Minic
18
6
11
1
11.5
4
Pirarucu
18
3
12
3
9
5
Gogobello
18
2
14
2
9
6
Marvin
18
3
11
4
8.5
7
Topple
18
3
11
4
8.5
8
Roc
18
1
14
3
8
9
Counter
18
0
11
7
5.5
Relegate to QL
10
iCE
18
0
11
7
5.5
Source:
League 2
League 2 saw the first time an NNUE (efficiently updatable neural network) engine played. NNUEs had been introduced to computer shogi in 2018, and defending champion Stockfish's developers had produced a working version for chess in June 2020. This new "Stockfish NNUE" gained strength very quickly, and by August 2020 was outperforming the original Stockfish by almost a hundred elo at short time controls.[6] This led to strong anticipation of the results when Igel, which had narrowly beaten Minic in League 3, submitted an updated version including an NNUE.
Igel lived up to the hype with a massive winning streak to start League 2. It scored 7/9 in the first round robin, including a win over hitherto-undefeated SlowChess. Its performance dipped in the second round, scoring "only" 4 wins and losing a game to eventual 3rd-place finisher RubiChess, but it still won the division with 14/18. Meanwhile SlowChess recovered after the loss to Igel by scoring 5 wins in the second round, losing no other games to finish second with 13/18. With its win against Igel, RubiChess was also the only engine to score 50% against the leader in the league, and finished clear 3rd with 11/18. The rest of the division was much more closely contested, and only two points separated 4th-placed Pedone from last-placed Nemorino.[1]
During testing for League 1, Booot, which had crashed in the previous season, needed to submit a patch addressing the crashes to participate. Its author did not submit an update, which led to Booot being disqualified and RubiChess promoting.[7]
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Igel
18
11
6
1
14
Advance to League 1
2
SlowChess
18
9
8
1
13
3
RubiChess
18
6
10
2
11
4
Pedone
18
3
11
4
8.5
5
Vajolet
18
4
8
6
8
6
Winter
18
3
10
5
8
7
Chiron
18
2
10
6
7
8
Demolito
18
2
10
6
7
9
Wasp
18
3
8
7
7
Relegate to League 3
10
Nemorino
18
1
11
6
6.5
Source:
League 1
League 1 was closely contested with the top eight engines only being separated by 2.5 points. Fire was able to secure its return to the Premier Division by starting strong in winning three of its first four games and then never relinquishing its lead. ScorpioNN started off by losing to RubiChess but recovered and moved into the promotion spot after winning its minimatches against Igel and rival Xiphos. SlowChess ended its impressive first season, which featured three straight promotions from the Qualification League to League One, by edging out Xiphos for third based on its better Sonneborn-Berger score. It missed promotion by only half a point. rofChade, which was relegated from the Premier Division last season, finished sixth but was the only engine that was able to defeat Fire. Igel, featuring NNUE, finished with an even score of 9/18 losing only one game to ScorpioNN, but it was also only able to win one game against rofChade. Fritz and Arasan, the only engines that weren't able to win any opening pairs, were demoted to League 2.[1]
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Fire
18
5
12
1
11
Advance to Premier Division
2
ScorpioNN
18
5
11
2
10.5
3
SlowChess
18
3
14
1
10
4
Xiphos
18
3
14
1
10
5
RubiChess
18
3
13
2
9.5
6
rofChade
18
3
13
2
9.5
7
Igel
18
1
16
1
9
8
Defenchess
18
2
13
3
8.5
9
Fritz
18
1
13
4
7.5
Relegate to League 2
10
Arasan
18
0
9
9
4.5
Source:
Premier Division
Komodo vs. Stockfish
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In the upset of the division, Komodo unleashed 18.Nxe6!, destroying Black's king position. The sacrifice caught Stockfish by surprise, and although it defended grimly, its king was too exposed for the position to be salvageable.
An error in the opening book used led to a first round robin with a very high draw rate. As a result, TCEC decided to add a fourth round robin, expanding the division to a quadruple double round robin.[8][9][10]
Wielding its new NNUE eval, Stockfish began the division as the strong favorite.[citation needed] However, in the first double round robin, it failed to live up to the hype. Instead, Leela took the lead with two wins while Stockfish only managed all draws. However, Stockfish reeled off four wins in the reverse round, defeating AllieStein, Ethereal, Fire and Komodo to seize the lead. As the division continued, Stockfish kept winning, but Leela kept pace, only a couple of wins behind – until Stockfish scored a head-to-head win in the fourth double round robin. This win, the first Premier Division win by Stockfish over Leela since TCEC Season 14, confirmed Stockfish's entry into the superfinal as the division winner. Leela finished second in spite of the loss as well, comfortably ahead of 3rd-placed Alliestein. The ordering of the top four were unchanged from Season 18, with Stoofvlees still finishing fourth in spite of not being updated.
In contrast to the top half of the table, which quickly sorted itself into its eventual order, the fight to avoid relegation was a topsy-turvy affair. Each engine fought to avoid losing to the engines in the top half – at the end of the division there were only four decisive games between the top four engines – while scoring wins against each other. Traditional powerhouse Komodo did badly in the first round, losing three games, and was last. Meanwhile, Ethereal started off in the lead with a win against Stoofvlees, but promptly lost the reverse, and eventually found itself in the cellar with Komodo after getting pummelled by Stockfish and Leela. Fire lost three games to Stockfish, but did only concede one loss to Leela and Alliestein, and in the third round robin piled the misery on Ethereal by scoring the first head-to-head win among the bottom four engines. Fellow promoted engine Scorpio lost three games to Leela, but was able to hold Stockfish to all draws, and even won an opening pair against Stoofvlees. Komodo was still in the relegation zone when it pulled off the upset of the division in round four, by defeating the runaway leader Stockfish (diagram). Although Stockish exacted revenge in the reverse, Komodo had handed Stockfish its only loss in the division, and with further wins against Scorpio and Fire, it finally finished fifth, 1.5 points above the relegation zone. Ethereal scored a win against Fire, but its fate was out of its hands, and when Scorpio won another game pair against Stoofvlees, its fate was sealed. In the end, Ethereal and Fire were relegated, 1.5 and 0.5 points away from safety respectively.[1]
Pos
Engine
Pld
W
D
L
Pts
Qualification
1
Stockfish
56
14
41
1
34.5
Advance to Superfinal
2
Leela Chess Zero
56
10
45
1
32.5
3
AllieStein
56
3
52
1
29
4
Stoofvlees
56
4
46
6
27
5
Komodo
56
4
45
7
26.5
6
ScorpioNN
56
2
47
7
25.5
7
Fire
56
2
46
8
25
Relegate to League 1
8
Ethereal
56
2
44
10
24
Source:
Superfinal
A poll of viewers conducted before the superfinal started found Stockfish to be a substantial favorite.[11]
The superfinal started with two pairs of traded wins before Stockfish took the lead in game 18, winning a Keres Attack after holding the reverse. Leela struck back in game 23 by winning a Queen's Indian defense. There followed a long streak of indecisive results of mostly draws, with Leela winning game 33 only to lose the reverse, until Stockfish took the lead by winning game 48. The relatively even first half was completely upended in the second, as Stockfish won one game pair after another. Although Leela won several games, it never managed to hold the reverse. Stockfish mathematically wrapped up the superfinal in games 93/94 by drawing game 93, then celebrated with another win in the reverse game. In the end, Stockfish won by 9 games (+18 -9 =73).[1]
In an article published shortly after the superfinal, GM Matthew Sadler wrote that although the superfinal had started as a nail-biter, Leela was ultimately unable to match Stockfish's conversion rate from superior positions, consigning it to defeat.
The following notes are selected from the many games analyzed by GM Sadler.[2]
Game 1
Leela Chess Zero vs. Stockfish, game 1
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Position after 22...Na4. Black attacks the b2-pawn. If White plays 23. c3, the White queen is cut off from the kingside, and allows Black Nc5-d3 ideas. Leela chooses instead to offer an exchange, but Stockfish refuses to bite.
Leela Chess Zero vs. Stockfish, game 1
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Position after 27. Rxe2. In spite of the seeming array of pieces defending White's king, Black has a perpetual after 27...Qg4+. If White attempts to run via the c-file, Black stops it with ...Rc8 and White cannot escape.
Game 1 was a mainline Sicilian Najdorf, one of the most deeply analyzed of openings, with the novelty of the game occurring only on move 21. Leela chose the 6. Be3 variation, and the game developed into a tense middlegame (diagram) in which Leela neutralized Stockfish's queenside play, but only at the cost of allowing Stockfish a perpetual check.
Position after 16. Rae1. After declaring a near-decisive -1.5 advantage earlier, Stockfish's evaluation crashed to 0.00 after 16...Bf5.
Leela Chess Zero vs. Stockfish, game 9 analysis
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Analysis diagram after 23.Qc3. White threatens d5+ and h3. 23...Bf6 24.Rxf6 Kxf6 25.h3 Qg5 26.h4 Qxh4 27.d5+ Kxf7 28.Rf1+ Kg8 29.Rf3 Rh7 30.Rg3+ Kf7 31.Rf3+ leads to a draw by perpetual check.
This game featured a Romantic line of the King's gambit, and yielded the first real evaluation disparity of the match. Playing White, Leela outcalculated Stockfish to draw the diagram position in spite of Stockfish's evaluation reaching -1.5 earlier in the game. Stockfish had originally intended 16...h5 17.Ne4 Bf5 18.exf6 Bh6 19.f7 Kg7, but apparently misjudged 20.Ng3, which leads to a draw after 20...Bg5 21.Nxf5+ Qxf5 22.Qg3 Qg4 23.Qc3 (see analysis diagram).
Book exit position. Stockfish played the surprising 8.Nxc6, which is usually considered by humans to be a positional mistake. In this particular position though, the kingside situation poses Black extra problems.
Stockfish vs. Leela Chess Zero, game 18
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1
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Position after 25...Bf4. With Black's king in the center, 26.Nd5+ is an obvious move. After 26...exd5 27.exd5+ Be5 28.Qh4, White threatens Rxe5+ followed by d6+, as well as advancing its passed h-pawn. Black's massed pieces against White's king look scary, but the counterplay is not fast enough.
In a line taken from the Karpov–Korchnoi world championship matches, both Stockfish and Leela took on c6 immediately after book exit, a move both GM Sadler and fellow kibitzing GM Danny Gormally were taught to be a positional mistake since it improved Black's center and also opened the b-file for Black's counterplay. However, in this particular position, White's advanced pawns on the kingside makes it hard for Black to find a home for its king. Leela kept its king in the center, but as Stockfish launched an attack in the middlegame, this poor king position turned out to be a decisive liability.
Position after 143.Bh4. White offers the b4-pawn. GM Sadler opined that a human would undoubtedly have smelled something fishy and refused, especially after White had failed to demonstrate a concrete plan earlier. Leela happily played 143...Rxb4, and was soon under the gun as the deflected rook meant Stockfish could mount a kingside attack without a Black rook to cover the 8th rank.
Stockfish vs. Leela Chess Zero, game 60
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With the incredible 150.Qh5, Stockfish breaks through decisively. If Leela had avoided 143...Rxb4, earlier, 143...Rg8 would have defended the position.
In this game featuring the Alekhine Defense, Stockfish had manoeuvred into a pleasant-but-blocked position in which it was unclear how it could make progress. After many moves of shuffling, Stockfish tried offering a pawn, which Leela unsuspectingly took and was promptly gunned down.
Position after 20.g4. With a wealth of attacking ideas at its disposal, Leela chooses a strong plan that caught GM Sadler by surprise.
Leela Chess Zero vs. Stockfish, game 77
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Position after 27.Bd2. Black is not mated, but is almost out of moves: moving the c8-rook allows Rac1 winning a piece; moving the f8-rook allows Bxf5, and moving the c7-knight loses a pawn to Bxb5. Stockfish desperately sacrifices a piece, but never came close to salvaging the game.
In this English opening, Stockfish castled into a dangerous kingside, after which Leela got to demonstrate it knew how to attack. The attack culminated in a bind in which Stockfish, despite sacrificing a piece in desperation, fails to wriggle out from.
Position after 25...Rc7. Black indirectly defends the g6-pawn because if White captures with 26.hxg6 hxg6 27.Rxg6, Black gains serious counterplay with 27...Rch7. Black simultaneously threatens to castle by hand on the queenside as well as to activate its light square bishop with 26...Bc8. With Black within sight of consolidating, Stockfish takes decisive measures by playing the pawn thrust 26.d6.
Stockfish vs. Leela Chess Zero, game 78
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Position after 30.Bc2. Having earlier sacrificed a pawn with 27.d7+ to stop Black from playing ...Bc8xf5, White intends Bb3+ to lock Black's king in the corner. With a subsequent h6, Black's king has no moves, which means it has to be very careful to guard against any major piece landing on the 8th rank.
Stockfish vs. Leela Chess Zero, game 78
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Position after 41.Ng3. Black has successfully eliminated White's menacing pawns on f5 and h6, but the pin on the c-file as well as the weak light squares on the kingside prove to be Black's downfall.
In the reverse game, Leela opted to keep its king in the center and concentrate on redeploying its pieces. It was within sight of consolidating when Stockfish unleashed an attack that broke through to win. GM Sadler called this game the best one of the superfinal, with both engines playing very well.
Position after 17...g6. Black threatens to develop its bishop followed by castling. Stockfish stops that plan with 18.Qg5, intending to meet 18...Bg7 with 19.Nh5. After 18...Ng8 (intending ...Bh6), Stockfish played 18.Qf4 Bh6 19.Ng5, this time immobilizing the g8-knight because of the weakness on f7.
Stockfish vs. Leela Chess Zero, game 92
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Position after 26.Rca1. White is in control of the board and Black's king position is awful. A move such as 26...Rhf8 would simply lose a piece to 27.Qh4+.
Stockfish vs. Leela Chess Zero, game 92
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Leela managed to liquidate into this position, but Stockfish demonstrated its "trapped" knight could still sacrifice itself for the e6-pawn to bring about a pawn avalanche that Black's bishop cannot stop.
In this French game, White's space advantage made it difficult for Black to develop its pieces. Leela resorted to drastic measures to free its position, but Stockfish deftly outplayed Leela in the ensuing complications to convert into a winning endgame.
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