# 2021 Open de Oeiras II – Singles

This was the second of four editions of the tournament in the 2021 ATP Challenger Tour calendar.
Zdeněk Kolář was the defending champion but chose not to defend his title.
Pedro Cachin won the title after defeating Nuno Borges 7–6$^{(7–4)}$, 7–6$^{(7–3)}$ in the final.

## Seeds

* Oscar Otte (second round)
* Enzo Couacaud (first round)
* Ernesto Escobedo (second round)
* Elliot Benchetrit (first round)
* Gian Marco Moroni (quarterfinals)
* Marco Trungelliti (first round)
* Nicola Kuhn (first round)
* Tristan Lamasine (first 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 |
| | Gonçalo Oliveira | 6$^4$ | 0$^r$ | | | | | |
| Q | Pedro Cachin | 7$^7$ | 4 | | | | | |
| Q | Pedro Cachin | 7$^7$ | 4 | | Q | Pedro Cachin | 7$^7$ | 7$^7$ |
| | | | | | WC | Nuno Borges | 6$^4$ | 6$^3$ |
| SE | Gastão Elias | 6 | 2 | 3 | WC | Nuno Borges | 6$^4$ | 6$^3$ |
| WC | Nuno Borges | 4 | 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 | Quarterfinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 1 | O Otte | 6 | 6 | | | | | | | | | | | | | | | |
| WC | Pedro Araújo | 2 | 4 | | 1 | O Otte | 4 | 6 | 3 | | | | | | | | | |
| | R Bonadio | 6$^4$ | 5 | | | G Oliveira | 6 | 4 | 6 | | | | | | | | | |
| | G Oliveira | 7$^7$ | 7 | | | | | | | | G Oliveira | 7 | 6 | | | | | |
| | M Zverev | 4 | | | | | | | | WC | T Cação | 5 | 2 | | | | | |
| | M Moraing | 3$^r$ | | | | M Zverev | 3 | 6 | 6$^6$ | | | | | | | | | |
| WC | T Cação | 6 | 6 | | WC | T Cação | 6 | 1 | 7$^8$ | | | | | | | | | |
| 8 | T Lamasine | 2 | 4 | | | | | | | | | | | | | G Oliveira | 6$^4$ | 0$^r$ |
| 4 | E Benchetrit | 2 | 1 | | | | | | | | | | | | Q | P Cachin | 7$^7$ | 4 |
| Q | P Cachin | 6 | 6 | | Q | P Cachin | 6 | 6 | | | | | | | | | | |
| | C-h Tseng | 6 | 6$^5$ | 3 | Q | A Andreev | 2 | 4 | | | | | | | | | | |
| Q | A Andreev | 1 | 7$^7$ | 6 | | | | | | Q | P Cachin | 6 | 2 | 6 | | | | |
| | P Kotov | 6 | 3 | | | | | | | 5 | GM Moroni | 3 | 6 | 4 | | | | |
| Alt | H Bourchier | 0 | 0$^r$ | | | P Kotov | 2 | 2 | | | | | | | | | | |
| | A Molčan | 2 | 4 | | 5 | GM Moroni | 6 | 6 | | | | | | | | | | |
| 5 | GM Moroni | 6 | 6 | | | | | | | | | | | | | | | |

### Bottom half

| 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 | M Trungelliti | 1 | 4 | | | | | | | | | | | | | | | |
| SE | G Elias | 6 | 6 | SE | G Elias | 6 | 6 | | | | | | | | | | | |
| | C Gimeno Valero | 6 | 6 | | C Gimeno Valero | 1 | 3 | | | | | | | | | | | |
| | R Ortega Olmedo | 1 | 1 | | | | | | SE | G Elias | w/o | | | | | | | |
| | D Kuzmanov | 6 | 6 | | | | | | | D Kuzmanov | | | | | | | | |
| | L Miedler | 1 | 0 | | D Kuzmanov | 6 | 6 | | | | | | | | | | | |
| Q | E Furness | 3 | 2 | 3 | E Escobedo | 2 | 2 | | | | | | | | | | | |
| 3 | E Escobedo | 6 | 6 | | | | | | | | | | | SE | G Elias | 6 | 2 | 3 |
| 7 | N Kuhn | 1 | 4 | | | | | | | | | | | WC | N Borges | 4 | 6 | 6 |
| | M Guinard | 6 | 6 | | M Guinard | 6 | 6 | | | | | | | | | | | |
| | H Grenier | 6 | 6 | | H Grenier | 4 | 1 | | | | | | | | | | | |
| | D-h Lee | 0 | 3 | | | | | | | M Guinard | 3 | 6 | 3 | | | | | |
| Alt | J Forejtek | 3$^r$ | | | | | | | WC | N Borges | 6 | 3 | 6 | | | | | |
| Q | R Brancaccio | 4 | | Q | R Brancaccio | 4 | 6 | 3 | | | | | | | | | | |
| WC | N Borges | 6 | 6 | WC | N Borges | 6 | 4 | 6 | | | | | | | | | | |
| 2 | E Couacaud | 3 | 4 | | | | | | | | | | | | | | | |