| Doubles | |
|---|---|
| 2021 Croatia Open Umag | |
| Champions |  Fernando Romboli  David Vega Hernández |
| Runners-up |  Tomislav Brkić  Nikola Ćaćić |
| Final score | 6–3, 7–5 |
Robin Haase and Philipp Oswald were the reigning champions from when the tournament was last held in 2019, [1] but chose not to defend their title.
Fernando Romboli and David Vega Hernández won the title, defeating Tomislav Brkić and Nikola Ćaćić in the final, 6–3, 7–5.
Tomislav Brkić / Nikola Ćaćić (final)
Pablo Cuevas / 
Fabrice Martin (first round) Rafael Matos / 
Andrei Vasilevski (first round)
Luis David Martínez / 
Artem Sitak (first round)| First round | Quarterfinals | Semifinals | Final | ||||||||||||||||||||||||
| 1 | T Brkić N Ćaćić | 6 | 3 | [10] | |||||||||||||||||||||||
| WC |  D Ajduković F Ninčević | 4 | 6 | [8] | 1 | T Brkić N Ćaćić | 6 | 6 | |||||||||||||||||||
 R Jebavý J Veselý | 7 | 6 | R Jebavý J Veselý | 2 | 2 | ||||||||||||||||||||||
| C Taberner B Zapata Miralles | 5 | 3 | 1 | T Brkić N Ćaćić | 6 | 6 | |||||||||||||||||||||
| 4 | LD Martinez A Sitak | 6 | 61 | [6] | I Sabanov M Sabanov | 1 | 2 | ||||||||||||||||||||
| I Sabanov M Sabanov | 3 | 77 | [10] | I Sabanov M Sabanov | 7 | 6 | |||||||||||||||||||||
 S Caruso G Mager | 78 | 2 | [6] | D Džumhur A Šančić | 5 | 0 | |||||||||||||||||||||
| D Džumhur A Šančić | 66 | 6 | [10] | 1 | T Brkić N Ćaćić | 3 | 5 | ||||||||||||||||||||
 R Albot P Martínez | 6 | 77 | F Romboli D Vega Hernández | 6 | 7 | ||||||||||||||||||||||
 A Begemann  I Zelenay | 4 | 65 | R Albot P Martínez | 6 | 6 | ||||||||||||||||||||||
| M Cecchinato S Travaglia | 7 | 6 | M Cecchinato S Travaglia | 4 | 1 | ||||||||||||||||||||||
| 3 | R Matos A Vasilevski | 5 | 4 | R Albot P Martínez | 6 | 68 | [7] | ||||||||||||||||||||
| PR | M López J Munar | 5 | 6 | [5] | F Romboli D Vega Hernández | 4 | 710 | [10] | |||||||||||||||||||
| F Romboli D Vega Hernández | 7 | 4 | [10] | F Romboli D Vega Hernández | 3 | 7 | [16] | ||||||||||||||||||||
| WC | A Kalender M Poljičak | 6 | 6 | WC | A Kalender M Poljičak | 6 | 5 | [14] | |||||||||||||||||||
| 2 | P Cuevas F Martin | 3 | 4 | ||||||||||||||||||||||||
Mateusz Kowalczyk and Andreas Mies were the defending champions but chose not to defend their title.
This was the first edition of the tournament.
Attila Balázs and Gonçalo Oliveira were the defending champions but chose not to defend their title.
Kevin Krawietz and Andreas Mies were the defending champions but chose not to defend their title.
Jurij Rodionov and Emil Ruusuvuori were the defending champions but chose not to defend their title.
Denys Molchanov and Igor Zelenay were the defending champions but chose not to defend their title.
Filippo Baldi and Andrea Pellegrino were the defending champions but chose not to defend their title.
Sander Gillé and Joran Vliegen were the defending champions but chose not to defend their title.
This was the first edition of the tournament.
Marcelo Demoliner and Matwé Middelkoop were the defending champions, but Middelkoop decide to participate in the 2021 Open Sud de France instead. Demoliner was pairing up with Santiago González, but they lost in the first round to Guillermo Durán and Andrés Molteni.
Marcel Granollers and Horacio Zeballos were the defending champions, but they decided not to participate this year.
This was the first edition of the tournament, primarily organised due to the cancellation of some tournaments in 2021, due to the COVID-19 pandemic.
This was the first of the three tournaments held in Belgrade for the 2021 tennis season. Belgrade also hosted a joint ATP/WTA tournament.
Oliver Marach and Mate Pavić were the two-time defending champions from when the event was last held in 2019 but lost in the first round to Tomislav Brkić and Nikola Ćaćić.
John Peers and Michael Venus were the defending champions, but Peers chose not to participate this year.
Austin Krajicek and Franko Škugor were the defending champions, but Krajicek chose to compete at the Summer Olympics and Škugor chose not to defend his title.
Robert Lindstedt and Jan-Lennard Struff were the reigning champions from when the tournament was last held in 2019, but chose not to participate this year.
Jamie Murray and Neal Skupski were the defending champions, but chose to compete in San Diego with different partners instead.
Marcelo Demoliner and Matwé Middelkoop were the reigning champions from when the tournament was last held in 2019, but chose to compete with different partners. Demoliner played alongside Marcus Daniell, but lost in the first round to Ariel Behar and Gonzalo Escobar.
Tomislav Brkić and Nikola Ćaćić were the defending champions but lost in the semifinals to Santiago González and Andrés Molteni.
| Grand Slam events | |
|---|---|
| ATP Tour Masters 1000 | |
| ATP Tour 500 | |
| ATP Tour 250 |
|
| Team events | |
 – Tournaments are affected by the COVID-19 pandemic * – Tournaments are introduced due to the impact of the COVID-19 pandemic | |