Stadium: Court C Round: 62 Ground: Clay
| ITF M25 Jablonec Nad Nisou | 2021/05/18 07:30 | Andrew Paulson v Kai Lemstra | 6-3,6-4 |
| ITF M25 Jablonec Nad Nisou | 2021/05/17 07:30 | Jan Jermar v Kai Lemstra | 4-6,4-6 |
| ITF M25 Jablonec Nad Nisou | 2021/05/16 08:30 | Matyas Cerny v Kai Lemstra | 3-6,3-6 |
| ITF M25 Meerbusch | 2021/04/12 09:30 | Marvin Netuschil v Kai Lemstra | 6-2,6-3 |
| ITF M15 Monastir | 2021/03/15 12:30 | Kai Lemstra v Alexander Kotzen | 1-6,3-6 |
| ITF M15 Monastir | 2021/03/15 08:00 | Kai Lemstra v John Sperle | 6-3,6-3 |
| ITF M15 Monastir | 2021/03/14 08:00 | Guy Orly Iradukunda v Kai Lemstra | 3-6,5-7 |
| ITF M15 Monastir | 2021/03/11 10:40 | Kai Lemstra v Juan Bautista Torres | 4-6,1-6 |
| Challenger Prague | 2021/05/02 14:40 | Vojtech Petr v Sergiy Stakhovsky | 3-6,7-6,5-7 |