# 2021 Slovak Open II – Singles

Maximilian Marterer was the defending champion but lost in the quarterfinals to Stefano Travaglia.
Tallon Griekspoor won the title after defeating Zsombor Piros 6–3, 6–2 in the final.

## Seeds

* Stefano Travaglia (semifinals, retired)
* Tallon Griekspoor (champion)
* Carlos Taberner (first round)
* Norbert Gombos (first round)
* Alex Molčan (semifinals)
* Gilles Simon (withdrew)
* Liam Broady (second round)
* Tomáš Macháč (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 |
| 1 | Stefano Travaglia | 6$^5$ | 2$^r$ | | | | | |
| Q | Zsombor Piros | 7$^7$ | 3 | | | | | |
| Q | Zsombor Piros | 7$^7$ | 3 | | Q | Zsombor Piros | 3 | 2 |
| | | | | | 2 | Tallon Griekspoor | 6 | 6 |
| 5 | Alex Molčan | 7$^7$ | 3 | 4 | 2 | Tallon Griekspoor | 6 | 6 |
| 2 | Tallon Griekspoor | 6$^5$ | 6 | 6 | | | | |

### Top half

| First round | First round | First round | First round | First round | Second round | Second round | Second round | Second round | Quarterfinals | Quarterfinals | Quarterfinals | Quarterfinals | Semifinals | Semifinals | Semifinals | Semifinals |
| 1 | S Travaglia | 7$^7$ | 6 | | | | | | | | | | | | | |
| | Z Kolář | 6$^4$ | 3 | | 1 | S Travaglia | 6 | 6 | | | | | | | | |
| | C İlkel | 6 | 6$^1$ | 6 | | C İlkel | 4 | 2 | | | | | | | | |
| LL | A Shevchenko | 4 | 7$^7$ | 3 | | | | | 1 | S Travaglia | 7$^7$ | 6 | | | | |
| | M Marterer | 4 | 6 | 6 | | | | | | M Marterer | 6$^1$ | 1 | | | | |
| | J Rodionov | 6 | 1 | 4 | | M Marterer | 6 | 7$^8$ | | | | | | | | |
| | I Marchenko | 2 | 1 | | 7 | L Broady | 4 | 6$^6$ | | | | | | | | |
| 7 | L Broady | 6 | 6 | | | | | | | | | | 1 | S Travaglia | 6$^5$ | 2$^r$ |
| 3 | C Taberner | 2 | 6$^5$ | | | | | | | | | | Q | Z Piros | 7$^7$ | 3 |
| | F Ferreira Silva | 6 | 7$^7$ | | | F Ferreira Silva | 6 | 7 | | | | | | | | |
| Q | D Svrčina | 2 | 7$^7$ | 6 | Q | D Svrčina | 4 | 5 | | | | | | | | |
| | F Horanský | 6 | 6$^5$ | 1 | | | | | | F Ferreira Silva | 0 | 4 | | | | |
| Q | M Karol | 4 | 3 | | | | | | Q | Z Piros | 6 | 6 | | | | |
| | L Lacko | 6 | 6 | | | L Lacko | 2 | 4 | | | | | | | | |
| Q | Z Piros | 7 | 3 | 6 | Q | Z Piros | 6 | 6 | | | | | | | | |
| 8 | T Macháč | 5 | 6 | 4 | | | | | | | | | | | | |

### 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 |
| 5 | A Molčan | 6 | 7$^9$ | | | | | | | | | | | | | | | | |
| | R Ramanathan | 3 | 6$^7$ | | 5 | A Molčan | 6 | 6 | | | | | | | | | | | |
| WC | J Forejtek | 2 | 4 | | Alt | T Kamke | 2 | 2 | | | | | | | | | | | |
| Alt | T Kamke | 6 | 6 | | | | | | | 5 | A Molčan | 6 | 4 | 6 | | | | | |
| | J Sousa | 6$^6$ | 7$^7$ | 6$^2$ | | | | | | | D Džumhur | 1 | 6 | 3 | | | | | |
| | A Çelikbilek | 7$^8$ | 6$^3$ | 7$^7$ | | A Çelikbilek | 4 | 4 | | | | | | | | | | | |
| | D Džumhur | 6 | 6 | | | D Džumhur | 6 | 6 | | | | | | | | | | | |
| 4 | N Gombos | 3 | 3 | | | | | | | | | | | | 5 | A Molčan | 7$^7$ | 3 | 4 |
| LL | S Mochizuki | 6$^5$ | 3 | | | | | | | | | | | | 2 | T Griekspoor | 6$^5$ | 6 | 6 |
| | K Żuk | 7$^7$ | 6 | | | K Żuk | 6 | 3 | 2 | | | | | | | | | | |
| | S Stakhovsky | 4 | 5 | | | J Lehečka | 2 | 6 | 6 | | | | | | | | | | |
| | J Lehečka | 6 | 7 | | | | | | | | J Lehečka | 6$^3$ | 4 | | | | | | |
| Q | D Kalenichenko | 7$^7$ | 7$^7$ | | | | | | | 2 | T Griekspoor | 7$^7$ | 6 | | | | | | |
| WC | Lukáš Palovič | 6$^4$ | 6$^5$ | | Q | D Kalenichenko | 6$^2$ | 6$^1$ | | | | | | | | | | | |
| WC | Lukáš Pokorný | 6$^4$ | 1 | | 2 | T Griekspoor | 7$^7$ | 7$^7$ | | | | | | | | | | | |
| 2 | T Griekspoor | 7$^7$ | 6 | | | | | | | | | | | | | | | | |