# 2021 San Marino Open – Singles

Adrian Ungur was the defending champion when the event was last held in 2014 but chose not to defend his title as he had retired from professional tennis.
Holger Rune won the title after defeating Orlando Luz 1–6, 6–2, 6–3 in the final.

## Seeds

* Marco Cecchinato (semifinals)
* Gilles Simon (second round)
* Salvatore Caruso (second round)
* Hugo Dellien (semifinals)
* Federico Gaio (first round)
* Tomás Martín Etcheverry (withdrew)
* Sebastián Báez (first round)
* Blaž Rola (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 | Final | Final | Final | Final | Final |
| 1 | Marco Cecchinato | 5 | 6$^4$ | | | | | |
| | Holger Rune | 7 | 7$^7$ | | | | | |
| | Holger Rune | 7 | 7$^7$ | | Holger Rune | 1 | 6 | 6 |
| | | | | Q | Orlando Luz | 6 | 2 | 3 |
| 4 | Hugo Dellien | 4 | 2 | Q | Orlando Luz | 6 | 2 | 3 |
| Q | Orlando Luz | 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 | M Cecchinato | 7$^7$ | 6 | | | | | | | | | | | | | | |
| | P Lorenzi | 6$^3$ | 2 | | 1 | M Cecchinato | 7$^7$ | 7$^8$ | | | | | | | | | |
| WC | R Brancaccio | 4 | 4 | | WC | L Nardi | 6$^4$ | 6$^6$ | | | | | | | | | |
| WC | L Nardi | 6 | 6 | | | | | | | 1 | M Cecchinato | 7$^7$ | 6 | | | | |
| | R Marcora | 7$^8$ | 7$^7$ | | | | | | | LL | P Kotov | 6$^5$ | 3 | | | | |
| | A Pellegrino | 6$^6$ | 6$^1$ | | | R Marcora | 4 | 4 | | | | | | | | | |
| LL | P Kotov | 7$^7$ | 7$^7$ | | LL | P Kotov | 6 | 6 | | | | | | | | | |
| 5 | F Gaio | 6$^2$ | 6$^3$ | | | | | | | | | | | 1 | M Cecchinato | 5 | 6$^4$ |
| 3 | S Caruso | 3 | 6 | 6 | | | | | | | | | | | H Rune | 7 | 7$^7$ |
| | Q Halys | 6 | 2 | 2 | 3 | S Caruso | 6 | 6$^4$ | 2 | | | | | | | | |
| PR | J Cagnina | 3 | 4 | | | H Rune | 2 | 7$^7$ | 6 | | | | | | | | |
| | H Rune | 6 | 6 | | | | | | | | H Rune | 6 | 7 | | | | |
| | J Menezes | 5 | 6 | 6 | | | | | | | J Menezes | 0 | 5 | | | | |
| | G Elias | 7 | 4 | 4 | | J Menezes | 6$^2$ | 7$^7$ | 7$^{11}$ | | | | | | | | |
| | GM Moroni | 6 | 3 | 5 | Q | F Forti | 7$^7$ | 6$^2$ | 6$^9$ | | | | | | | | |
| Q | F Forti | 4 | 6 | 7 | | | | | | | | | | | | | |

### 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 | Semifinals | Semifinals | Semifinals | Semifinals |
| 7 | S Báez | 6$^4$ | 7 | 5 | | | | | | | | | | | | | |
| | J Clarke | 7$^7$ | 5 | 7 | | J Clarke | | | | | | | | | | | |
| | B Gojo | 5 | 2 | | | F Meligeni Alves | w/o | | | | | | | | | | |
| | F Meligeni Alves | 7 | 6 | | | | | | | | F Meligeni Alves | 3 | 2 | | | | |
| | R Olivo | 6$^4$ | 4 | | | | | | | 4 | H Dellien | 6 | 6 | | | | |
| | M Copil | 7$^7$ | 6 | | | M Copil | 6 | 5 | 4 | | | | | | | | |
| Q | Manuel Mazza | 4 | 5 | | 4 | H Dellien | 4 | 7 | 6 | | | | | | | | |
| 4 | H Dellien | 6 | 7 | | | | | | | | | | | 4 | H Dellien | 4 | 2 |
| 8 | B Rola | 6 | 6 | | | | | | | | | | | Q | O Luz | 6 | 6 |
| | Z Kolář | 3 | 3 | | 8 | B Rola | 6 | 4 | 2 | | | | | | | | |
| Alt | M Jaziri | 3 | 2$^r$ | | Q | O Luz | 4 | 6 | 6 | | | | | | | | |
| Q | O Luz | 6 | 4 | | | | | | | Q | O Luz | 7$^9$ | 7 | | | | |
| | M Janvier | 6 | 6 | | | | | | | | M Janvier | 6$^7$ | 5 | | | | |
| Q | J Ocleppo | 2 | 3 | | | M Janvier | 7 | 6 | | | | | | | | | |
| WC | M De Rossi | 3 | 3 | | 2 | G Simon | 5 | 1 | | | | | | | | | |
| 2 | G Simon | 6 | 6 | | | | | | | | | | | | | | |