| Singles | |
|---|---|
| 2018 Sparkassen ATP Challenger | |
| Champion |  Ugo Humbert |
| Runner-up | Pierre-Hugues Herbert |
| Final score | 6–4, 6–2 |
Lorenzo Sonego was the defending champion but lost in the first round to Constant Lestienne.
Ugo Humbert won the title after defeating Pierre-Hugues Herbert 6–4, 6–2 in the final.
Pierre-Hugues Herbert (Final)
Lukáš Lacko (First round)
Jaume Munar (First round)
Lorenzo Sonego (First round)
Ričardas Berankis (Quarterfinals) Ugo Humbert (Champion)
Yannick Maden (Withdrew) Matthias Bachinger (First round)| Semifinals | Final | ||||||||||||
| 1 | Pierre-Hugues Herbert | 77 | 6 | ||||||||||
 Dennis Novak | 65 | 4 | |||||||||||
| 1 | Pierre-Hugues Herbert | 4 | 2 | ||||||||||
| 6 | Ugo Humbert | 6 | 6 | ||||||||||
| 6 | Ugo Humbert | 6 | 66 | 6 | |||||||||
| Grégoire Barrère | 4 | 78 | 3 | ||||||||||
| First Round | Second Round | Quarterfinals | Semifinals | ||||||||||||||||||||||||
| 1 | P-H Herbert | 7 | 6 | ||||||||||||||||||||||||
 T Brkić | 5 | 4 | 1 | P-H Herbert | 78 | 6 | |||||||||||||||||||||
| WC | L Caruana | 4 | 710 | 6 | WC | L Caruana | 66 | 4 | |||||||||||||||||||
| S Robert | 6 | 68 | 2 | 1 | P-H Herbert | 6 | 6 | ||||||||||||||||||||
| D Masur | 6 | 4 | 64 | 5 | R Berankis | 3 | 2 | ||||||||||||||||||||
| O Otte | 0 | 6 | 77 | O Otte | 2 | 7 | 2 | ||||||||||||||||||||
| Q | Tobias Simon | 65 | 4 | 5 | R Berankis | 6 | 5 | 6 | |||||||||||||||||||
| 5 | R Berankis | 77 | 6 | 1 | P-H Herbert | 77 | 6 | ||||||||||||||||||||
| 3 | J Munar | 3 | 63 | D Novak | 65 | 4 | |||||||||||||||||||||
| Q |  Kirill Kivattsev | 6 | 77 | Q | K Kivattsev | 3 | 2 | ||||||||||||||||||||
 S Stakhovsky | 64 | 3 | S Bolelli | 6 | 6 | ||||||||||||||||||||||
| S Bolelli | 77 | 6 | S Bolelli | 65 | 65 | ||||||||||||||||||||||
| S Caruso | 2 | 4 | D Novak | 77 | 77 | ||||||||||||||||||||||
| D Brown | 6 | 6 | D Brown | 2 | 7 | 4 | |||||||||||||||||||||
| D Novak | 1 | 6 | 77 | D Novak | 6 | 5 | 6 | ||||||||||||||||||||
| 8 | M Bachinger | 6 | 3 | 61 | |||||||||||||||||||||||
| First Round | Second Round | Quarterfinals | Semifinals | ||||||||||||||||||||||||
| 6 | U Humbert | 6 | 6 | ||||||||||||||||||||||||
| Q | B Hassan | 4 | 3 | 6 | U Humbert | 64 | 6 | 6 | |||||||||||||||||||
| LL | Fabrizio Ornago | 65 | 4 | Q Halys | 77 | 3 | 4 | ||||||||||||||||||||
| Q Halys | 77 | 6 | 6 | U Humbert | 715 | 6 | |||||||||||||||||||||
| WC | J Sinner | 77 | 6 | C Lestienne | 613 | 3 | |||||||||||||||||||||
| WC | Luca Giacomini | 64 | 1 | WC | J Sinner | 78 | 5 | 5 | |||||||||||||||||||
| C Lestienne | 6 | 4 | 6 | C Lestienne | 66 | 7 | 7 | ||||||||||||||||||||
| 4 | L Sonego | 1 | 6 | 4 | 6 | U Humbert | 6 | 66 | 6 | ||||||||||||||||||
| Q | K Krawietz | 77 | 4 | 77 | G Barrère | 4 | 78 | 3 | |||||||||||||||||||
| A Hoang | 64 | 6 | 65 | Q | K Krawietz | 3 | 5 | ||||||||||||||||||||
| M Moraing | 6 | 6 | M Moraing | 6 | 7 | ||||||||||||||||||||||
| WC | R Brancaccio | 2 | 2 | M Moraing | 6 | 1 | 2 | ||||||||||||||||||||
| G Barrère | 7 | 66 | 6 | G Barrère | 4 | 6 | 6 | ||||||||||||||||||||
| S Napolitano | 5 | 78 | 2 | G Barrère | 77 | 77 | |||||||||||||||||||||
 A Pavić | 79 | 718 | A Pavić | 61 | 63 | ||||||||||||||||||||||
| 2 | L Lacko | 67 | 616 | ||||||||||||||||||||||||
Stefano Napolitano was the defending champion but lost in the second round to Matteo Donati.
Blaž Kavčič was the defending champion but chose not to defend his title.
Norbert Gombos was the defending champion but lost in the first round to Luca Vanni.
Yannick Hanfmann was the defending champion but chose not to defend his title.
Corentin Moutet was the defending champion but lost in the second round to Julien Benneteau.
Uladzimir Ignatik was the defending champion but lost in the second round to Ugo Humbert.
Aljaž Bedene was the defending champion but chose not to defend his title.
Pablo Andújar was the defending champion, but lost in the final to Benoît Paire, 2–6, 3–6.
Rafael Nadal was the three-time defending champion, but lost in the semifinals to Fabio Fognini.
Alexander Zverev was the defending champion but lost in the quarterfinals to Stefanos Tsitsipas.
Borna Ćorić was the defending champion, but retired from his quarterfinal match against Pierre-Hugues Herbert.
Damir Džumhur was the defending champion, but retired from his quarterfinal match against Jordan Thompson.
John Isner was the two-time defending champion and had won the title five times in the last six years, but he lost in the second round to Reilly Opelka. This was only the second time out of ten in the Atlanta Open's history that Isner failed to reach the singles final.
Daniil Medvedev was the defending champion, but chose not to participate this year.
Dominic Thiem was the defending champion, but lost to Andrey Rublev in the quarterfinals.
The Murray River Open was a new addition to the ATP Tour in 2021.
Gaël Monfils was the defending champion, but chose not to defend his title.
Daniil Medvedev defeated Pierre-Hugues Herbert in the final, 6–4, 6–7(4–7), 6–4 to capture the Men's Singles tennis title at the 2021 Open 13.
Hubert Hurkacz defeated Jannik Sinner in the final, 7–6(7–4), 6–4 to win the 2021 Miami Open Men's Singles tennis title. Hurkacz claimed his first Masters 1000 singles championship and became the first Polish man to win a Masters 1000 Singles event.
Alexander Zverev defeated Matteo Berrettini in the final, 6–7(8–10), 6–4, 6–3 to win the Men's Singles tennis title at the 2021 Madrid Open.