Stadium: Court 2 Round: 26 Ground: Clay
| ITF M15 - Frankfurt Am Main | 2023/08/08 09:00 | Kai Lemstra v Matthew William Donald | 6-3,6-1 |
| ITF M15 - Metzingen | 2023/07/28 10:50 | Kai Lemstra v Sander Jong | 6-3,0-6,2-6 |
| ITF M15 - Metzingen | 2023/07/27 15:00 | Kai Lemstra v Juan C P Angelo | 7-5,6-0 |
| ITF M15 - Metzingen | 2023/07/26 12:30 | Stefanos Sakellaridis v Kai Lemstra | 3-6,3-6 |
| ITF M15 - Tanger | 2023/06/08 11:00 | Kai Lemstra v Constantin Bittoun Kouzmine | 3-6,3-6 |
| ITF M15 - Tanger | 2023/06/07 14:00 | Maik Steiner v Kai Lemstra | 1-6,5-7 |
| ITF M15 - Constanta | 2023/05/30 11:00 | Simon A Ivanov v Kai Lemstra | 6-2,6-2 |
| ITF M15 - Warmbad-Villach | 2023/05/23 07:30 | Kai Lemstra v Giovanni Oradini | 3-6,6-7 |
| Challenger - Porto | 2023/08/01 16:10 | Johannes Haerteis v Emilio Nava | 6-7,6-7 |
| Challenger - Porto | 2023/07/31 10:00 | Hugo Maia v Johannes Haerteis | 5-7,5-7 |
| Challenger - Porto | 2023/07/30 10:00 | Johannes Haerteis v Victor Lilov | 6-4,2-6,6-2 |
| ITF M25 - Porto | 2023/07/27 11:45 | Johannes Haerteis v Joao Domingues | 6-2,6-7,2-6 |
| ITF M25 - Porto | 2023/07/26 11:35 | Tomas Pinho v Johannes Haerteis | 4-6,6-7 |
| ITF M15 - Cluj Napoca | 2023/06/22 06:50 | Johannes Haerteis v Gabi Adrian Boitan | 2-6,3-6 |
| ITF M15 - Cluj Napoca | 2023/06/21 09:10 | Dan Alexandru Tomescu v Johannes Haerteis | 4-6,2-6 |
| ITF M15 - Cluj Napoca | 2023/06/20 06:25 | Johannes Haerteis v Mihai Alexandru Coman | 6-4,7-6 |