Stadium: Center Court Round: 25 Ground: Hardcourt outdoor
| Challenger Iasi | 2021/07/12 11:50 | Felipe Meligeni Rodrigues Alves v Igor Sijsling | 7-5,6-2 |
| Challenger - Braunschweig | 2021/07/04 10:10 | Igor Sijsling v Diego Hidalgo | 3-6,6-3,4-6 |
| Challenger - Forli | 2021/06/16 11:15 | Mats Moraing v Igor Sijsling | 3-6,6-4,6-2 |
| Challenger - Forli | 2021/06/14 15:40 | Igor Sijsling v Andrea Pellegrino | 6-2,7-6 |
| Challenger - Nottingham | 2021/06/07 12:10 | Aleksandar Vukic v Igor Sijsling | 6-4,7-6 |
| Challenger - Nottingham | 2021/06/06 18:30 | Igor Sijsling v Matteo Viola | 6-7,6-1,6-0 |
| Challenger - Oeiras | 2021/05/25 11:30 | Igor Sijsling v Timofey Skatov | 6-2,2-6,6-7 |
| Challenger - Rom | 2021/04/27 09:30 | Danilo Petrovic v Igor Sijsling | 6-1,7-6 |
| ATP - Hamborg | 2021/07/12 10:30 | Nicola Kuhn v Thiago Seyboth Wild | 6-1,1-6,2-6 |
| Challenger - Braunschweig | 2021/07/07 11:40 | Nicola Kuhn v Henri Laaksonen | 4-6,5-7 |
| Challenger - Braunschweig | 2021/07/06 15:25 | Nicola Kuhn v Mischa Zverev | 6-2,7-6 |
| Challenger - Porto | 2021/06/29 12:50 | Nicola Kuhn v Emilio Gomez | 5-7,2-6 |
| ATP - Mallorca | 2021/06/21 17:10 | Nicola Kuhn v Feliciano Lopez | 1-6,6-7 |
| ATP - Mallorca | 2021/06/20 11:30 | Radu Albot v Nicola Kuhn | 6-7,4-6 |
| ATP - Mallorca | 2021/06/19 09:00 | Nicola Kuhn v Pedro Martinez | 6-2,6-2 |
| ATP - Halle | 2021/06/12 10:15 | Nicola Kuhn v Daniel Elahi Galan | 6-7,6-4,6-7 |