# 2021 St. Petersburg Open – Singles

Andrey Rublev was the defending champion, but lost in the quarterfinals to Botic van de Zandschulp.
Marin Čilić won the title, defeating Taylor Fritz in the final, 7–6$^{(7–3)}$, 4–6, 6–4.

## Seeds

The top four seeds received a bye into the second round.
* Andrey Rublev (quarterfinals)
* Denis Shapovalov (quarterfinals)
* Roberto Bautista Agut (quarterfinals)
* Aslan Karatsev (second round)
* Taylor Fritz (final)
* Karen Khachanov (second round)
* Alexander Bublik (second round)
* Sebastian Korda (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 | Final |
| Q | Botic van de Zandschulp | 3 | 3 | | | | | | |
| | Marin Čilić | 6 | 6 | | | | | | |
| | Marin Čilić | 6 | 6 | | | Marin Čilić | 7$^7$ | 4 | 6 |
| | | | | | 5 | Taylor Fritz | 6$^3$ | 6 | 4 |
| 5 | Taylor Fritz | 5 | 6 | 6 | 5 | Taylor Fritz | 6$^3$ | 6 | 4 |
| | Jan-Lennard Struff | 7 | 1 | 3 | | | | | |

### Top 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 |
| | | | | 1 | A Rublev | 6 | 6 | | | | | | | | | | |
| | L Đere | 3 | 2 | | I Ivashka | 4 | 4 | | | | | | | | | | |
| | I Ivashka | 6 | 6 | | | | | | 1 | A Rublev | 3 | 4 | | | | | |
| Q | B van de Zandschulp | 6 | 6 | | | | | | Q | B van de Zandschulp | 6 | 6 | | | | | |
| Q | Y Nishioka | 2 | 2 | Q | B van de Zandschulp | 6 | 7 | | | | | | | | | | |
| WC | N Serdarušić | 6$^2$ | 5 | 8 | S Korda | 2 | 5 | | | | | | | | | | |
| 8 | S Korda | 7$^7$ | 7 | | | | | | | | | | | Q | B van de Zandschulp | 3 | 3 |
| | | | | | | | | | | | | | | | M Čilić | 6 | 6 |
| | | | | 3 | R Bautista Agut | 6 | 6 | | | | | | | | | | |
| Q | E Gerasimov | 5 | 4 | | M McDonald | 3 | 2 | | | | | | | | | | |
| | M McDonald | 7 | 6 | | | | | | 3 | R Bautista Agut | 4 | 6 | 3 | | | | |
| | M Čilić | 6 | 6 | | | | | | | M Čilić | 6 | 3 | 6 | | | | |
| | A Ramos Viñolas | 2 | 3 | | M Čilić | 6 | 6$^1$ | 6 | | | | | | | | | |
| | A Mannarino | 3 | 3 | 6 | K Khachanov | 4 | 7$^7$ | 4 | | | | | | | | | |
| 6 | K Khachanov | 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 | Semifinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 5 | T Fritz | 6 | 6 | | | | | | | | | | | | | | |
| Q | E Ruusuvuori | 1 | 4 | 5 | T Fritz | 7$^7$ | 6 | | | | | | | | | | |
| | P Martínez | 3 | 2 | | T Paul | 6$^3$ | 4 | | | | | | | | | | |
| | T Paul | 6 | 6 | | | | | | 5 | T Fritz | 6 | 6 | | | | | |
| | J Millman | 6 | 6 | | | | | | | J Millman | 4 | 2 | | | | | |
| WC | Y Oliel | 1 | 1 | | J Millman | 6 | 6 | | | | | | | | | | |
| | | | | 4 | A Karatsev | 3 | 2 | | | | | | | | | | |
| | | | | | | | | | | | | | 5 | T Fritz | 5 | 6 | 6 |
| 7 | A Bublik | 6 | 6 | | | | | | | | | | | J-L Struff | 7 | 1 | 3 |
| WC | E Tiurnev | 3 | 4 | 7 | A Bublik | 4 | 3 | | | | | | | | | | |
| | J-L Struff | 7$^7$ | 6 | | J-L Struff | 6 | 6 | | | | | | | | | | |
| | J Duckworth | 6$^3$ | 3 | | | | | | | J-L Struff | 6 | 6 | | | | | |
| | F Delbonis | 2 | 6$^1$ | | | | | | 2 | D Shapovalov | 4 | 3 | | | | | |
| PR | P Andújar | 6 | 7$^7$ | PR | P Andújar | 6 | 3 | 0 | | | | | | | | | |
| | | | | 2 | D Shapovalov | 2 | 6 | 6 | | | | | | | | | |

## Qualifying
### Seeds

* Botic van de Zandschulp (qualified)
* Benjamin Bonzi (first round)
* Federico Coria (first round)
* Miomir Kecmanović (qualifying competition)
* Emil Ruusuvuori (qualified)
* Yoshihito Nishioka (qualified)
* Mikael Ymer (first round)
* Egor Gerasimov (qualified)

### Qualifiers

* Botic van de Zandschulp
* Emil Ruusuvuori
* Egor Gerasimov
* Yoshihito Nishioka

### Qualifying draw
#### First qualifier

| First round | First round | First round | First round | Qualifying competition | Qualifying competition | Qualifying competition | Qualifying competition | Qualifying competition |
| 1 | Botic van de Zandschulp | 6 | | | | | | |
| Alt | Marc Polmans | 6$^r$ | | | | | | |
| Alt | Marc Polmans | 6$^r$ | | 1 | Botic van de Zandschulp | 5 | 6 | 6 |
| | | | | Alt | Evgeny Donskoy | 7 | 3 | 2 |
| Alt | Evgeny Donskoy | 6 | 7 | Alt | Evgeny Donskoy | 7 | 3 | 2 |
| 7 | Mikael Ymer | 2 | 5 | | | | | |

#### Second qualifier

| First round | First round | First round | First round | First round | Qualifying competition | Qualifying competition | Qualifying competition | Qualifying competition |
| 2 | Benjamin Bonzi | 6 | 6$^4$ | 2 | | | | |
| WC | Dmitry Popko | 4 | 7$^7$ | 6 | | | | |
| WC | Dmitry Popko | 4 | 7$^7$ | 6 | WC | Dmitry Popko | 6$^2$ | 4 |
| | | | | | 5 | Emil Ruusuvuori | 7$^7$ | 6 |
| PR | Gilles Simon | 0 | 3 | | 5 | Emil Ruusuvuori | 7$^7$ | 6 |
| 5 | Emil Ruusuvuori | 6 | 6 | | | | | |

#### Third qualifier

| First round | First round | First round | First round | Qualifying competition | Qualifying competition | Qualifying competition | Qualifying competition |
| 3 | Federico Coria | 3 | 4 | | | | |
| | Radu Albot | 6 | 6 | | | | |
| | Radu Albot | 6 | 6 | | Radu Albot | 6$^2$ | 4 |
| | | | | 8 | Egor Gerasimov | 7$^7$ | 6 |
| | Damir Džumhur | 1 | 6$^6$ | 8 | Egor Gerasimov | 7$^7$ | 6 |
| 8 | Egor Gerasimov | 6 | 7$^8$ | | | | |

#### Fourth qualifier

| First round | First round | First round | First round | First round | Qualifying competition | Qualifying competition | Qualifying competition | Qualifying competition | Qualifying competition |
| 4 | Miomir Kecmanović | 2 | 6 | 6 | | | | | |
| WC | Evgeny Karlovskiy | 6 | 4 | 1 | | | | | |
| WC | Evgeny Karlovskiy | 6 | 4 | 1 | 4 | Miomir Kecmanović | 3 | 6 | 2 |
| | | | | | 6 | Yoshihito Nishioka | 6 | 4 | 6 |
| Alt | Roman Safiullin | 5 | 2 | | 6 | Yoshihito Nishioka | 6 | 4 | 6 |
| 6 | Yoshihito Nishioka | 7 | 6 | | | | | | |