# 2021 Lisboa Belém Open – Men's singles

Jaume Munar was the defending champion but chose not to defend his title.
Dmitry Popko won the title after defeating Andrea Pellegrino 6–2, 6–4 in the final.

## Seeds

* 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)

## Draw
### Key

* Q = Qualifier
* WC = Wild card
* LL = Lucky loser
* Alt = Alternate
* SE = Special exempt
* PR = Protected ranking
* ITF = ITF entry
* JE = Junior exempt
* w/o = Walkover
* r = Retired
* d = Defaulted
* SR = Special ranking

### Finals

| Semifinals | Semifinals | Semifinals | Semifinals | Final | Final | Final | Final |
| | Andrea Pellegrino | 7$^7$ | 1 | | | | |
| 4 | Andrej Martin | 6$^5$ | 0$^r$ | | | | |
| 4 | Andrej Martin | 6$^5$ | 0$^r$ | | Andrea Pellegrino | 2 | 4 |
| | | | | 8 | Dmitry Popko | 6 | 6 |
| 3 | Hugo Gaston | 3 | 4 | 8 | Dmitry Popko | 6 | 6 |
| 8 | Dmitry Popko | 6 | 6 | | | | |

### Top half

| First round | First round | First round | First round | First round | Second round | Second round | Second round | Second round | Second round | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 1 | T Monteiro | 6 | 6 | | | | | | | | | | | | | | |
| | A Müller | 4 | 4 | | 1 | T Monteiro | 3 | 6$^8$ | | | | | | | | | |
| | I Sijsling | 0 | 4 | | | A Pellegrino | 6 | 7$^{10}$ | | | | | | | | | |
| | 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 | 6$^3$ | 6 | 6 | | | | | | | | |
| | G Zeppieri | 4 | 6 | 6 | | G Zeppieri | 7$^7$ | 4 | 0 | | | | | | | | |
| Q | S Rodríguez Taverna | 6 | 3 | 4 | | | | | | | | | | | A Pellegrino | 7$^7$ | 1 |
| 4 | A Martin | 4 | 6 | 6 | | | | | | | | | | 4 | A Martin | 6$^5$ | 0$^r$ |
| | R Cid Subervi | 6 | 4 | 0 | 4 | A Martin | 2 | 6 | 6 | | | | | | | | |
| WC | G Oliveira | 7$^7$ | 6 | | WC | G Oliveira | 6 | 4 | 3 | | | | | | | | |
| WC | Pedro Araújo | 6$^4$ | 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 | | | | | | | | | | | | | | |

### Bottom half

| First round | First round | First round | First round | First round | Second round | Second round | Second round | Second round | Second round | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 6 | C-M Stebe | 6 | 6 | | | | | | | | | | | | | | | |
| LL | J Berrettini | 0 | 1 | | 6 | C-M Stebe | 4 | 7$^7$ | 6 | | | | | | | | | |
| | M Geerts | 6 | 3 | 4 | | N Borges | 6 | 6$^4$ | 4 | | | | | | | | | |
| | N Borges | 4 | 6 | 6 | | | | | | 6 | C-M Stebe | 6$^3$ | 3$^r$ | | | | | |
| | R Haase | 6 | 4 | 6 | | | | | | 3 | H Gaston | 7$^7$ | 4 | | | | | |
| | F Baldi | 4 | 6 | 4 | | R Haase | 6 | 6$^5$ | 3 | | | | | | | | | |
| | F Forti | 3 | 1 | | 3 | H Gaston | 4 | 7$^7$ | 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 | 1$^r$ | 8 | D Popko | 6 | 6 | | | | | | | | | | |
| | A Arnaboldi | 2 | 6 | 6$^8$ | Q | B Hassan | 2 | 4 | | | | | | | | | | |
| Q | B Hassan | 6 | 3 | 7$^{10}$ | | | | | | 8 | D Popko | 6$^4$ | 7 | 6 | | | | |
| | J Domingues | 6 | 6 | | | | | | | | J Domingues | 7$^7$ | 5 | 0 | | | | |
| WC | T Cação | 4 | 4 | | | J Domingues | 6 | 6 | | | | | | | | | | |
| Q | D Michalski | 7$^7$ | 5 | 6 | Q | D Michalski | 2 | 2 | | | | | | | | | | |
| 2 | T Daniel | 6$^4$ | 7 | 2 | | | | | | | | | | | | | | |