| Men's singles | |
|---|---|
| 2021 Lisboa Belém Open | |
| Champion |  Dmitry Popko |
| Runner-up |  Andrea Pellegrino |
| Score | 6–2, 6–4 |
Jaume Munar was the defending champion [1] but chose not to defend his title.
Dmitry Popko won the title after defeating Andrea Pellegrino 6–2, 6–4 in the final.
Thiago Monteiro (second round)
Taro Daniel (first round)
Hugo Gaston (semifinals)
Andrej Martin (semifinals, retired)
Nikola Milojević (quarterfinals)
Cedrik-Marcel Stebe (quarterfinals, retired) Alessandro Giannessi (withdrew) Dmitry Popko (champion)| Semifinals | Final | ||||||||||||
| Andrea Pellegrino | 77 | 1 | |||||||||||
| 4 | Andrej Martin | 65 | 0r | ||||||||||
| Andrea Pellegrino | 2 | 4 | |||||||||||
| 8 | Dmitry Popko | 6 | 6 | ||||||||||
| 3 | Hugo Gaston | 3 | 4 | ||||||||||
| 8 | Dmitry Popko | 6 | 6 | ||||||||||
| First round | Second round | Quarterfinals | Semifinals | ||||||||||||||||||||||||
| 1 | T Monteiro | 6 | 6 | ||||||||||||||||||||||||
| A Müller | 4 | 4 | 1 | T Monteiro | 3 | 68 | |||||||||||||||||||||
 I Sijsling | 0 | 4 | A Pellegrino | 6 | 710 | ||||||||||||||||||||||
| A Pellegrino | 6 | 6 | A Pellegrino | 6 | 6 | ||||||||||||||||||||||
 F Ferreira Silva | 6 | 6 | F Ferreira Silva | 3 | 1 | ||||||||||||||||||||||
| T Lamasine | 3 | 4 | F Ferreira Silva | 63 | 6 | 6 | |||||||||||||||||||||
| G Zeppieri | 4 | 6 | 6 | G Zeppieri | 77 | 4 | 0 | ||||||||||||||||||||
| Q |  S Rodríguez Taverna | 6 | 3 | 4 | A Pellegrino | 77 | 1 | ||||||||||||||||||||
| 4 | A Martin | 4 | 6 | 6 | 4 | A Martin | 65 | 0r | |||||||||||||||||||
 R Cid Subervi | 6 | 4 | 0 | 4 | A Martin | 2 | 6 | 6 | |||||||||||||||||||
| WC | G Oliveira | 77 | 6 | WC | G Oliveira | 6 | 4 | 3 | |||||||||||||||||||
| WC | Pedro Araújo | 64 | 4 | 4 | A Martin | 6 | 6 | ||||||||||||||||||||
| PR |  E Benchetrit | 2 | 2 | 5 | N Milojević | 4 | 4 | ||||||||||||||||||||
| C Lestienne | 6 | 6 | C Lestienne | 5 | 4 | ||||||||||||||||||||||
| Q | S Fanselow | 4 | 4 | 5 | N Milojević | 7 | 6 | ||||||||||||||||||||
| 5 | N Milojević | 6 | 6 | ||||||||||||||||||||||||
| First round | Second round | Quarterfinals | Semifinals | ||||||||||||||||||||||||
| 6 | C-M Stebe | 6 | 6 | ||||||||||||||||||||||||
| LL | J Berrettini | 0 | 1 | 6 | C-M Stebe | 4 | 77 | 6 | |||||||||||||||||||
 M Geerts | 6 | 3 | 4 | N Borges | 6 | 64 | 4 | ||||||||||||||||||||
| N Borges | 4 | 6 | 6 | 6 | C-M Stebe | 63 | 3r | ||||||||||||||||||||
| R Haase | 6 | 4 | 6 | 3 | H Gaston | 77 | 4 | ||||||||||||||||||||
| F Baldi | 4 | 6 | 4 | R Haase | 6 | 65 | 3 | ||||||||||||||||||||
| F Forti | 3 | 1 | 3 | H Gaston | 4 | 77 | 6 | ||||||||||||||||||||
| 3 | H Gaston | 6 | 6 | 3 | H Gaston | 3 | 4 | ||||||||||||||||||||
| 8 | D Popko | 1 | 6 | 4 | 8 | D Popko | 6 | 6 | |||||||||||||||||||
| L Giustino | 6 | 3 | 1r | 8 | D Popko | 6 | 6 | ||||||||||||||||||||
| A Arnaboldi | 2 | 6 | 68 | Q | B Hassan | 2 | 4 | ||||||||||||||||||||
| Q | B Hassan | 6 | 3 | 710 | 8 | D Popko | 64 | 7 | 6 | ||||||||||||||||||
| J Domingues | 6 | 6 | J Domingues | 77 | 5 | 0 | |||||||||||||||||||||
| WC | T Cação | 4 | 4 | J Domingues | 6 | 6 | |||||||||||||||||||||
| Q |  D Michalski | 77 | 5 | 6 | Q | D Michalski | 2 | 2 | |||||||||||||||||||
| 2 | T Daniel | 64 | 7 | 2 | |||||||||||||||||||||||
Teymuraz Gabashvili was the defending champion, but decided not to defend his title.
Mikhail Kukushkin was the defending champion, but chose not to defend his title.
Yuki Bhambri was the defending champion but chose not to defend his title.
Karen Khachanov was the defending champion but chose not to defend his title.
Marco Cecchinato was the defending champion but lost in the semifinals to Federico Delbonis.
Jordan Thompson was the defending champion but chose not to defend his title.
Alexander Bublik was the defending champion but chose not to defend his title.
Yannick Hanfmann was the defending champion but chose not to defend his title.
Alessandro Giannessi was the defending champion but chose not to defend his title.
Juan Pablo Varillas was the defending champion but chose not to defend his title.
Lorenzo Giustino was the defending champion but lost in the first round to Timofey Skatov.
Mikael Ymer was the defending champion but chose not to defend his title.
Tommy Robredo was the defending champion but chose not to defend his title.
Adam Pavlásek was the defending champion but chose not to defend his title.
John Millman was the defending champion, but lost in the quarterfinals to James Duckworth.
Gastão Elias was the two-time defending champion and successfully defended his title, defeating Alessandro Giannessi 7–6(7–4), 6–1 in the final.
Tallon Griekspoor was the defending champion but chose not to defend his title.
Andrey Kuznetsov was the defending champion but chose not to defend his title.
Benjamin Bonzi was the defending champion but chose not to defend his title.
Dmitry Popko was the defending champion but chose not to defend his title.