# 2021 Zagreb Open – Singles

This was the first edition of the tournament since 2011.
Sebastián Báez won the title after defeating Juan Pablo Varillas 3–6, 6–3, 6–1 in the final.

## Seeds

* Federico Coria (second round)
* Pedro Martínez (quarterfinals)
* Andrej Martin (second round)
* Yasutaka Uchiyama (first round)
* Pedro Sousa (first round)
* Benjamin Bonzi (first round)
* Damir Džumhur (second round)
* Thiago Seyboth Wild (quarterfinals)

## 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 |
| | Juan Manuel Cerúndolo | 1$^r$ | | | | | | |
| | Juan Pablo Varillas | 5 | | | | | | |
| | Juan Pablo Varillas | 5 | | | Juan Pablo Varillas | 6 | 3 | 1 |
| | | | | Q | Sebastián Báez | 3 | 6 | 6 |
| Q | Marcelo Tomás Barrios Vera | 3 | 4 | Q | Sebastián Báez | 3 | 6 | 6 |
| Q | Sebastián Báez | 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 |
| 1 | F Coria | 6 | 7$^9$ | | | | | | | | | | | | |
| | M Safwat | 4 | 6$^7$ | | 1 | F Coria | 6$^4$ | 2 | | | | | | | |
| | JM Cerúndolo | 6 | 6 | | | JM Cerúndolo | 7$^7$ | 6 | | | | | | | |
| | E Donskoy | 3 | 2 | | | | | | | JM Cerúndolo | 6 | 6$^3$ | 6 | | |
| | B van de Zandschulp | 6 | 3 | 7 | | | | | | B van de Zandschulp | 4 | 7$^7$ | 2 | | |
| | M Mmoh | 4 | 6 | 5 | | B van de Zandschulp | 6 | 2 | 6 | | | | | | |
| | A Tabilo | 6 | 6 | | | A Tabilo | 1 | 6 | 4 | | | | | | |
| 6 | B Bonzi | 4 | 1 | | | | | | | | | | | JM Cerúndolo | 1$^r$ |
| 4 | Y Uchiyama | 3 | 2 | | | | | | | | | | | JP Varillas | 5 |
| | N Milojević | 6 | 6 | | | N Milojević | 7$^7$ | 6 | | | | | | | |
| | L Mayer | 6 | 6$^4$ | 6 | | L Mayer | 6$^4$ | 2 | | | | | | | |
| | F Ferreira Silva | 3 | 7$^7$ | 3 | | | | | | N Milojević | 6$^3$ | 3 | | | |
| WC | Antun Vidak | 0 | 1 | | | | | | | JP Varillas | 7$^7$ | 6 | | | |
| | B Rola | 6 | 6 | | | B Rola | 4 | 2 | | | | | | | |
| | JP Varillas | 7$^7$ | 7 | | | JP Varillas | 6 | 6 | | | | | | | |
| 5 | P Sousa | 6$^4$ | 5 | | | | | | | | | | | | |

### 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 |
| 8 | T Seyboth Wild | 0 | 7 | 7 | | | | | | | | | | | | | | |
| | R Safiullin | 6 | 5 | 5 | 8 | T Seyboth Wild | 7 | 6 | | | | | | | | | | |
| Q | E Gulbis | 6$^5$ | 6$^5$ | | WC | B Gojo | 5 | 4 | | | | | | | | | | |
| WC | B Gojo | 7$^7$ | 7$^7$ | | | | | | | 8 | T Seyboth Wild | 4 | 2 | | | | | |
| Alt | D Popko | 4 | 2 | | | | | | | Q | MT Barrios Vera | 6 | 6 | | | | | |
| Q | MT Barrios Vera | 6 | 6 | | Q | MT Barrios Vera | 2 | 6 | 6 | | | | | | | | | |
| | J Jung | 6 | 6$^4$ | 0$^r$ | 3 | A Martin | 6 | 3 | 2 | | | | | | | | | |
| 3 | A Martin | 2 | 7$^7$ | 4 | | | | | | | | | | | Q | MT Barrios Vera | 3 | 4 |
| 7 | D Džumhur | 6 | 6 | | | | | | | | | | | | Q | S Báez | 6 | 6 |
| | G Soeda | 4 | 3 | | 7 | D Džumhur | 7$^7$ | 1 | 5 | | | | | | | | | |
| Q | S Báez | 6 | 6 | | Q | S Báez | 6$^5$ | 6 | 7 | | | | | | | | | |
| Q | R Ramanathan | 4 | 0 | | | | | | | Q | S Báez | 3 | 6 | 6 | | | | |
| | K Żuk | 4 | 6 | 6 | | | | | | 2 | P Martínez | 6 | 3 | 4 | | | | |
| | J Rodionov | 6 | 3 | 3 | | K Żuk | 6$^6$ | 2 | | | | | | | | | | |
| WC | N Serdarušić | 4 | 7$^7$ | 3 | 2 | P Martínez | 7$^8$ | 6 | | | | | | | | | | |
| 2 | P Martínez | 6 | 6$^1$ | 6 | | | | | | | | | | | | | | |