Stadium: Court 12 Round: 26 Ground: Clay
| Challenger Forli MD | 2021/06/15 14:20 | Lenz/Moraing v Sijsling / Smits | 4-6,7-6,10-1 |
| Challenger St. Petersburg MD | 2021/03/11 11:00 | Sijsling / Smits v Kravchuk/Yevseyev | 2-6,4-6 |
| Challenger St. Petersburg MD | 2021/03/10 10:00 | Gabashvili/Nedovyesov v Sijsling / Smits | 1-6,6-3,4-10 |
| Challenger - Cherbourg - Doubles Hommes | 2021/02/11 14:45 | Sijsling / Smits v Klein L / Molcan A | 6-7,6-7 |
| Challenger - Cherbourg - Doubles Hommes | 2021/02/09 14:45 | Drzewiecki/Jebavy v Sijsling / Smits | 6-1,6-7,7-10 |
| ITF - M15 Bressuire - Doubles Hommes | 2021/01/19 14:55 | de Schepper/Napolitano v Sijsling / Smits | 7-6,6-7,10-5 |
| ITF - M15 Antalya - Doubles Hommes | 2021/01/09 11:45 | Sijsling / Smits v Harris/Sabanin | 6-0,3-6,13-15 |
| ITF - M15 Antalya - Doubles Hommes | 2021/01/09 08:35 | Sijsling / Smits v Deschamps/Savelev | 6-2,7-5 |