# 2021 Open de Oeiras III – Singles

Pedro Cachin was the defending champion but chose to not defend his title.
Carlos Alcaraz won the title after defeating Facundo Bagnis 6–4, 6–4 in the final.

## Seeds

* Jiří Veselý (second round, retired)
* Steve Johnson (first round)
* Federico Coria (second round)
* Pedro Martínez (second round)
* Yannick Hanfmann (first round)
* Juan Ignacio Londero (first round)
* Facundo Bagnis (final)
* Pedro Sousa (quarterfinals)

## 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 | Semifinals | Final | Final | Final | Final |
| | Taro Daniel | 2 | 7 | 2 | | | | |
| | Carlos Alcaraz | 6 | 5 | 6 | | | | |
| | Carlos Alcaraz | 6 | 5 | 6 | | Carlos Alcaraz | 6 | 6 |
| | | | | | 7 | Facundo Bagnis | 4 | 4 |
| 7 | Facundo Bagnis | 6 | 6 | | 7 | Facundo Bagnis | 4 | 4 |
| | Hugo Gaston | 0 | 2 | | | | | |

### 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 | Quarterfinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 1 | J Veselý | 6 | 7$^7$ | | | | | | | | | | | | | | | |
| Q | H Grenier | 4 | 6$^2$ | | 1 | J Veselý | 3 | 3$^r$ | | | | | | | | | | |
| | S Ofner | 1 | 6$^4$ | | | H Dellien | 6 | 4 | | | | | | | | | | |
| | H Dellien | 6 | 7$^7$ | | | | | | | | H Dellien | 1 | 6 | 1 | | | | |
| | B Rola | 1 | 5 | | | | | | | | T Daniel | 6 | 3 | 6 | | | | |
| | T Daniel | 6 | 7 | | | T Daniel | 6 | 6 | | | | | | | | | | |
| | L Mayer | 4 | 6 | 6 | | L Mayer | 3 | 4 | | | | | | | | | | |
| 6 | JI Londero | 6 | 1 | 4 | | | | | | | | | | | T Daniel | 2 | 7 | 2 |
| 3 | F Coria | 2 | 6 | 6 | | | | | | | | | | | C Alcaraz | 6 | 5 | 6 |
| Q | A Molčan | 6 | 4 | 1 | 3 | F Coria | 4 | 6 | 4 | | | | | | | | | |
| | M Cressy | 6$^3$ | 4 | | WC | G Elias | 6 | 4 | 6 | | | | | | | | | |
| WC | G Elias | 7$^7$ | 6 | | | | | | | WC | G Elias | 5 | 4 | | | | | |
| | C Alcaraz | 6 | 6 | | | | | | | | C Alcaraz | 7 | 6 | | | | | |
| | Y Uchiyama | 0 | 3 | | | C Alcaraz | 3 | 6 | 6 | | | | | | | | | |
| | B Nakashima | 6 | 6 | | | B Nakashima | 6 | 4 | 4 | | | | | | | | | |
| 5 | Y Hanfmann | 3 | 4 | | | | | | | | | | | | | | | |

### 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 |
| 7 | F Bagnis | 7 | 3 | | | | | | | | | | | | | | | |
| | C Taberner | 5 | 2$^r$ | | 7 | F Bagnis | 2 | 6 | 6 | | | | | | | | | |
| | J Jung | 6 | 4 | 3 | Alt | F Ferreira Silva | 6 | 4 | 2 | | | | | | | | | |
| Alt | F Ferreira Silva | 1 | 6 | 6 | | | | | | 7 | F Bagnis | 6 | 5 | 7 | | | | |
| WC | N Borges | 6 | 6 | | | | | | | WC | N Borges | 1 | 7 | 5 | | | | |
| | B Zapata Miralles | 4 | 4 | | WC | N Borges | 6 | 6 | | | | | | | | | | |
| WC | J Domingues | 4 | 2 | | 4 | P Martínez | 2 | 4 | | | | | | | | | | |
| 4 | P Martínez | 6 | 6 | | | | | | | | | | | | 7 | F Bagnis | 6 | 6 |
| 8 | P Sousa | 3 | 7$^7$ | 4 | | | | | | | | | | | | H Gaston | 0 | 2 |
| | D Džumhur | 6 | 6$^4$ | 0$^r$ | 8 | P Sousa | 6 | 6 | | | | | | | | | | |
| | D Kudla | 4 | 6 | 5 | | B van de Zandschulp | 3 | 2 | | | | | | | | | | |
| | B van de Zandschulp | 6 | 3 | 7 | | | | | | 8 | P Sousa | 3 | 6 | 2 | | | | |
| | H Gaston | 7$^7$ | 6 | | | | | | | | H Gaston | 6 | 1 | 6 | | | | |
| Q | Z Bergs | 6$^1$ | 1 | | | H Gaston | 6 | 6 | | | | | | | | | | |
| Q | E Gómez | 7$^7$ | 3 | 6 | Q | E Gómez | 4 | 3 | | | | | | | | | | |
| 2 | S Johnson | 6$^4$ | 6 | 3 | | | | | | | | | | | | | | |