Stadium: Centre Court Round: 25 Ground: Clay
| ITF M25 Prostejov | 2021/08/17 09:15 | Martin Krumich v Daniel Siniakov | 3-6,6-4,7-5 |
| Challenger Liberec | 2021/08/03 15:10 | Nikola Milojevic v Daniel Siniakov | 7-5,6-0 |
| Challenger Liberec | 2021/08/02 10:35 | Daniel Siniakov v David Vega Hernandez | 6-3,7-5 |
| Challenger Liberec | 2021/08/01 16:30 | Geoffrey Blancaneaux v Daniel Siniakov | 2-6,3-6 |
| ITF M15 Poprad | 2021/07/06 07:55 | Illya Beloborodko v Daniel Siniakov | 3-6,7-6,7-5 |
| ITF M15 Poprad | 2021/07/05 08:00 | Calin Manda v Daniel Siniakov | 3-6,0-6 |
| ITF M15 Poprad | 2021/07/04 11:00 | Lukas Pokorny v Daniel Siniakov | 6-2,4-6,5-10 |
| ITF M25 Wroclaw | 2021/06/29 08:00 | Daniel Siniakov v Illya Beloborodko | 6-4,4-6,6-10 |
| Challenger Segovia | 2021/07/27 14:25 | Igor Sijsling v Teymuraz Gabashvili | 1-6,3-6 |
| Challenger Pozoblanco | 2021/07/22 16:00 | Tim Van Rijthoven v Igor Sijsling | 6-4,6-4 |
| Challenger Iasi | 2021/07/12 11:50 | Felipe Meligeni Rodrigues Alves v Igor Sijsling | 7-5,6-2 |
| Challenger Braunschweig | 2021/07/04 10:10 | Igor Sijsling v Diego Hidalgo | 3-6,6-3,4-6 |
| Challenger Forli | 2021/06/16 11:15 | Mats Moraing v Igor Sijsling | 3-6,6-4,6-2 |
| Challenger Forli | 2021/06/14 15:40 | Igor Sijsling v Andrea Pellegrino | 6-2,7-6 |
| Challenger Nottingham | 2021/06/07 12:10 | Aleksandar Vukic v Igor Sijsling | 6-4,7-6 |
| Challenger Nottingham | 2021/06/06 18:30 | Igor Sijsling v Matteo Viola | 6-7,6-1,6-0 |