# 2021 Open de Oeiras – Singles

This was the first of four editions of the tournament in the 2021 ATP Challenger Tour calendar.
Zdeněk Kolář won the title after defeating Gastão Elias 6–4, 7–5 in the final.

## Seeds

* Oscar Otte (first round)
* Enzo Couacaud (first round)
* Ramkumar Ramanathan (withdrew)
* Ernesto Escobedo (first round)
* Andrea Collarini (first round)
* Elliot Benchetrit (first round)
* Gian Marco Moroni (second round)
* Zdeněk Kolář (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 |
| | Marco Trungelliti | 3 | 2 | | | | |
| 8 | Zdeněk Kolář | 6 | 6 | | | | |
| 8 | Zdeněk Kolář | 6 | 6 | 8 | Zdeněk Kolář | 6 | 7 |
| | | | | WC | Gastão Elias | 4 | 5 |
| WC | Gastão Elias | 6 | 7 | WC | Gastão Elias | 4 | 5 |
| Q | Evan Furness | 3 | 5 | | | | |

### 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 | O Otte | 3 | 4 | | | | | | | | | | | | | | |
| | M Trungelliti | 6 | 6 | | | M Trungelliti | 6 | 6 | | | | | | | | | |
| | G Oliveira | 6 | 6 | | | G Oliveira | 1 | 3 | | | | | | | | | |
| Q | D Yevseyev | 1 | 2 | | | | | | | | M Trungelliti | 6 | 6 | | | | |
| Q | M Guinard | 1 | 5 | | | | | | | | I Sijsling | 3 | 4 | | | | |
| | I Sijsling | 6 | 7 | | | I Sijsling | 6 | 6 | | | | | | | | | |
| | A Vatutin | 4 | 3 | | 7 | GM Moroni | 2 | 2 | | | | | | | | | |
| 7 | GM Moroni | 6 | 6 | | | | | | | | | | | | M Trungelliti | 3 | 2 |
| LL | F Cabral | 2 | 7$^7$ | 2 | | | | | | | | | | 8 | Z Kolář | 6 | 6 |
| | M Moraing | 6 | 6$^5$ | 6 | | M Moraing | 6 | 2 | 5 | | | | | | | | |
| | D-h Lee | 3 | 4 | | WC | T Cação | 4 | 6 | 7 | | | | | | | | |
| WC | T Cação | 6 | 6 | | | | | | | WC | T Cação | 3 | 2 | | | | |
| Alt | H Bourchier | 5 | 4 | | | | | | | 8 | Z Kolář | 6 | 6 | | | | |
| | L Miedler | 7 | 6 | | | L Miedler | 1 | 1 | | | | | | | | | |
| WC | N Borges | 6 | 6$^6$ | 1 | 8 | Z Kolář | 6 | 6 | | | | | | | | | |
| 8 | Z Kolář | 2 | 7$^8$ | 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 |
| 5 | A Collarini | 6 | 3 | 5 | | | | | | | | | | | | | | |
| | A Molčan | 3 | 6 | 7 | | A Molčan | 6 | 6 | | | | | | | | | | |
| | N Serdarušić | 6 | 6 | | | N Serdarušić | 4 | 2 | | | | | | | | | | |
| LL | Luís Faria | 3 | 3 | | | | | | | | A Molčan | 2 | 5 | | | | | |
| | C-h Tseng | 3 | 6 | 4 | | | | | | WC | G Elias | 6 | 7 | | | | | |
| WC | G Elias | 6 | 2 | 6 | WC | G Elias | 3 | 6 | 6 | | | | | | | | | |
| | R Ortega Olmedo | 7 | 6 | | | R Ortega Olmedo | 6 | 1 | 3 | | | | | | | | | |
| 4 | E Escobedo | 5 | 4 | | | | | | | | | | | | WC | G Elias | 6 | 7 |
| 6 | E Benchetrit | 3 | 1 | | | | | | | | | | | | Q | E Furness | 3 | 5 |
| Q | R Brancaccio | 6 | 6 | | Q | R Brancaccio | 6 | 6 | | | | | | | | | | |
| | N Kuhn | 6 | 6 | | | N Kuhn | 1 | 2 | | | | | | | | | | |
| | T Gabashvili | 3 | 3 | | | | | | | Q | R Brancaccio | 1 | 6 | 4 | | | | |
| | K Uchida | 6 | 4 | 6$^3$ | | | | | | Q | E Furness | 6 | 1 | 6 | | | | |
| | R Bonadio | 3 | 6 | 7$^7$ | | R Bonadio | 6 | 3 | 4 | | | | | | | | | |
| Q | E Furness | 6 | 3 | 7$^7$ | Q | E Furness | 4 | 6 | 6 | | | | | | | | | |
| 2 | E Couacaud | 4 | 6 | 6$^4$ | | | | | | | | | | | | | | |