Stadium: Sparkassen Court Round: 54 Ground: Clay
| ITF M25 Haag | 2021/06/29 15:00 | Dayne Kelly v Guy Den Heijer | 6-1,6-4 |
| ITF M25 Haag | 2021/06/28 09:40 | Guy Den Heijer v Max Van Nunen | 6-3,6-1 |
| ITF M25 Haag | 2021/06/27 08:00 | Guy Den Heijer v Jing L Weng | 6-3,6-4 |
| ITF M25 Alkmaar | 2021/06/23 09:00 | Kaichi Uchida v Guy Den Heijer | 7-6,6-3 |
| ITF M25 Alkmaar | 2021/06/21 18:00 | Vladimir Ivanov v Guy Den Heijer | 3-6,6-7 |
| ITF M25 Alkmaar | 2021/06/20 17:00 | Alec Deckers v Guy Den Heijer | 3-6,7-6,4-10 |
| ITF M25 Vic | 2021/05/16 08:00 | Mateus Alves v Guy Den Heijer | 6-2,6-3 |
| ITF M15 Valldoreix | 2021/05/09 13:00 | Guy Den Heijer v Pol Amoros Ramos | 3-6,2-6 |