| UTR Pro Tennis Series Mulheim | 2021/09/20 09:30 | Colin Sinclair v Edison Ambarzumjan | 7-5,5-4 |
| UTR Pro Tennis Series Sofia | 2021/09/03 07:00 | George Lazarov v Keisidis, Christos | 6-4,6-4 |
| UTR Pro Tennis Series Sofia | 2021/09/02 07:00 | Vladimir Popov v Keisidis, Christos | 7-6,6-7,3-6 |
| UTR Pro Tennis Series Sofia | 2021/09/01 08:00 | Keisidis, Christos v Mario Radev | 6-1,4-6,6-4 |
| UTR Pro Tennis Series Sofia | 2021/08/31 08:00 | Keisidis, Christos v Josh Sheehy | 4-6,4-6 |
| UTR Pro Tennis Series Sofia | 2021/08/30 07:00 | Keisidis, Christos v Prodanov, Denislav | 6-1,5-1 |
| ITF - M15 Héraklion | 2021/06/09 08:00 | March, Antonio Cayetano v Keisidis, Christos | 4-6,7-6,6-2 |
| ITF - M15 Héraklion | 2021/06/08 06:30 | Sean Hodkin v Keisidis, Christos | 6-4,5-7,4-10 |
| ITF - M15 Héraklion | 2021/06/07 09:00 | Keisidis, Christos v Gaponenko, Alexander | 6-4,6-1 |