| German Championship | 2021/12/07 15:00 | Kai Lemstra v Tom Schonenberg | 3-6,0-6 |
| ITF M15 Heraklion | 2021/11/23 07:00 | Nicholas David Ionel v Kai Lemstra | 3-6,7-5,10-4 |
| ITF M15 Heraklion | 2021/11/22 08:00 | Sergis Kyratzis v Kai Lemstra | 4-6,2-6 |
| ITF M15 Heraklion | 2021/11/21 07:00 | Liam Puttergill v Kai Lemstra | 2-6,1-6 |
| ITF M15 Heraklion | 2021/11/17 08:05 | Alastair Gray v Kai Lemstra | 6-1,7-5 |
| ITF M15 Heraklion | 2021/11/16 07:00 | Max Houkes v Kai Lemstra | 4-6,6-7 |
| ITF M15 Heraklion | 2021/11/15 08:00 | Alan Gleeson v Kai Lemstra | 6-7,3-6 |
| ITF M15 Heraklion | 2021/11/12 10:05 | Kai Lemstra v Peter Fajta | 6-4,1-6,2-6 |
| Challenger Lisbon | 2021/09/27 15:10 | Igor Sijsling v Andrea Pellegrino | 0-6,4-6 |
| Challenger Braga | 2021/09/21 09:00 | Igor Sijsling v Michael Vrbensky | 1-6,4-6 |
| Challenger Prague | 2021/08/27 09:00 | Franco Agamenone v Igor Sijsling | 6-3,6-0 |
| Challenger Prague | 2021/08/26 10:00 | Igor Sijsling v Kaichi Uchida | 7-5,6-3 |
| Challenger Prague | 2021/08/24 13:45 | Daniel Siniakov v Igor Sijsling | 4-6,3-6 |
| Challenger Segovia | 2021/07/27 14:25 | Igor Sijsling v Teymuraz Gabashvili | 1-6,3-6 |
| Challenger Pozoblanco | 2021/07/22 16:00 | Tim Van Rijthoven v Igor Sijsling | 6-4,6-4 |
| Challenger Iasi | 2021/07/12 11:50 | Felipe Meligeni Rodrigues Alves v Igor Sijsling | 7-5,6-2 |