# 2021 Open de Oeiras IV – Singles

Carlos Alcaraz was the defending champion but chose not to defend his title.
Gastão Elias won the title after defeating Holger Rune 5–7, 6–4, 6–4 in the final.

## Seeds

* Mathias Bourgue (first round)
* Wu Tung-lin (second round)
* Julian Lenz (second round)
* Tseng Chun-hsin (second round)
* Roberto Quiroz (first round)
* Carlos Gimeno Valero (quarterfinals)
* Pedro Cachin (first round)
* Riccardo Bonadio (second round)

## 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 |
| | Gastão Elias | 4 | 7 | 0 | | | | |
| | Nuno Borges | 6 | 5 | 0$^r$ | | | | |
| | Nuno Borges | 6 | 5 | 0$^r$ | Gastão Elias | 5 | 6 | 6 |
| | | | | | Holger Rune | 7 | 4 | 4 |
| | Holger Rune | 6 | 3 | 6 | Holger Rune | 7 | 4 | 4 |
| Q | Timofey Skatov | 2 | 6 | 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 | M Bourgue | 2 | 2 | | | | | | | | | | | | | | | |
| WC | Pedro Araújo | 6 | 6 | | WC | P Araújo | 5 | 0 | | | | | | | | | | |
| | G Elias | 6 | 6 | | | G Elias | 7 | 6 | | | | | | | | | | |
| WC | Luís Faria | 2 | 3 | | | | | | | | G Elias | 6$^0$ | 6 | 6 | | | | |
| | C Ugo Carabelli | 6 | 6 | | | | | | | | C Ugo Carabelli | 7$^7$ | 3 | 0 | | | | |
| | J de Jong | 4 | 2 | | | C Ugo Carabelli | 4 | 7 | 6 | | | | | | | | | |
| | J Forejtek | 6$^4$ | 6 | 0 | 8 | R Bonadio | 6 | 5 | 4 | | | | | | | | | |
| 8 | R Bonadio | 7$^7$ | 3 | 6 | | | | | | | | | | | G Elias | 4 | 7 | 0 |
| 4 | C-h Tseng | 6 | 6 | | | | | | | | | | | | N Borges | 6 | 5 | 0$^r$ |
| | R Ortega Olmedo | 4 | 2 | | 4 | C-h Tseng | 3 | 6 | 1 | | | | | | | | | |
| | J Härteis | 3 | 0 | | Q | FC Jianu | 6 | 3 | 6 | | | | | | | | | |
| Q | FC Jianu | 6 | 6 | | | | | | | Q | FC Jianu | 4 | 4 | | | | | |
| | N Serdarušić | 4 | 5 | | | | | | | | N Borges | 6 | 6 | | | | | |
| Q | D Yevseyev | 6 | 7 | | Q | D Yevseyev | 7$^7$ | 4 | 2 | | | | | | | | | |
| | N Borges | 6 | 6 | | | N Borges | 6$^2$ | 6 | 6 | | | | | | | | | |
| 5 | R Quiroz | 3 | 0 | | | | | | | | | | | | | | | |

### 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 | Semifinals |
| 6 | C Gimeno Valero | 6 | 6 | | | | | | | | | | | | | | | | |
| | V Sachko | 2 | 1 | | 6 | C Gimeno Valero | 6 | 6 | | | | | | | | | | | |
| PR | V Galović | 4 | 7$^7$ | 6 | PR | V Galović | 4 | 2 | | | | | | | | | | | |
| | D Ajduković | 6 | 6$^4$ | 1 | | | | | | 6 | C Gimeno Valero | 2 | 4 | | | | | | |
| | H Rune | 6 | 6 | | | | | | | | H Rune | 6 | 6 | | | | | | |
| | T van Rijthoven | 3 | 4 | | | H Rune | 5 | 6 | 6 | | | | | | | | | | |
| Alt | C Denolly | 4 | 0 | | 3 | J Lenz | 7 | 4 | 3 | | | | | | | | | | |
| 3 | J Lenz | 6 | 6 | | | | | | | | | | | | | H Rune | 6 | 3 | 6 |
| 7 | P Cachin | 4 | 7 | 6$^1$ | | | | | | | | | | | Q | T Skatov | 2 | 6 | 2 |
| | G Oliveira | 6 | 5 | 7$^7$ | | G Oliveira | 3 | 1 | | | | | | | | | | | |
| | I Sijsling | 6 | 2 | 6$^4$ | Q | T Skatov | 6 | 6 | | | | | | | | | | | |
| Q | T Skatov | 2 | 6 | 7$^7$ | | | | | | Q | T Skatov | 7$^7$ | 5 | 6 | | | | | |
| | G Blancaneaux | 3 | 2 | | | | | | | Q | N Sánchez Izquierdo | 6$^4$ | 7 | 1 | | | | | |
| Q | N Sánchez Izquierdo | 6 | 6 | | Q | N Sánchez Izquierdo | 6 | 7$^7$ | | | | | | | | | | | |
| WC | T Cação | 1 | 1 | | 2 | T-l Wu | 2 | 6$^2$ | | | | | | | | | | | |
| 2 | T-l Wu | 6 | 6 | | | | | | | | | | | | | | | | |