Stadium: M (CC) Round: 27 Ground: Clay
| UTR Pro Tennis Series Mulheim | 2021/07/30 13:30 | Guy Den Heijer v Alexander Hoogmartens | 6-2,3-6,4-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/30 13:30 | Guy Den Heijer v Alexander Hoogmartens | 6-2,3-6,4-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/29 13:30 | Mark Wallner v Guy Den Heijer | 2-6,3-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/29 07:00 | Matthew Thomson v Guy Den Heijer | 1-6,3-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/27 09:30 | Billy Harris v Guy Den Heijer | 3-6,2-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/26 11:00 | Guy Den Heijer v Tim Sandkaulen | 7-5,6-3 |
| ITF M25 Marburg | 2021/07/07 10:50 | Aleksandre Metreveli v Guy Den Heijer | 6-3,1-6,6-2 |
| ITF M25 Marburg | 2021/07/06 12:25 | Guy Den Heijer v Florian Broska | 6-3,7-6 |
| ITF M25 Marburg | 2021/07/05 08:00 | Guy Den Heijer v Maximilian Homberg | 6-7,6-2,10-8 |
| UTR Pro Tennis Series Mulheim | 2021/07/31 08:30 | Vasile Antonescu v Alexander Hoogmartens | 5-7,7-6,6-1 |
| UTR Pro Tennis Series Mulheim | 2021/07/30 13:30 | Guy Den Heijer v Alexander Hoogmartens | 6-2,3-6,4-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/30 07:00 | Francesco Vilardo v Alexander Hoogmartens | 1-6,2-5 |
| UTR Pro Tennis Series Mulheim | 2021/07/29 08:30 | Jim Hendrikx v Alexander Hoogmartens | 4-6,3-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/28 12:30 | Stefan Peter Hampe v Alexander Hoogmartens | 2-6,1-6 |
| UTR Pro Tennis Series Mulheim | 2021/07/26 08:00 | Hazem Naw v Alexander Hoogmartens | 6-4,5-4 |
| ITF M15 Gaiba | 2021/06/09 16:15 | Alexander Hoogmartens v Federico Bertuccioli | 7-6,2-6,4-6 |
| ITF M15 Antalya | 2021/03/17 09:55 | Miljan Zekic v Alexander Hoogmartens | 5-7,7-5,6-2 |