Stadium: Pista 3 Round: 25 Ground: Hardcourt outdoor
| Challenger Mallorca | 2021/08/31 08:00 | Nicolas Moreno De Alboran v Haerteis, Johannes | 3-6,6-4,4-6 |
| Challenger Mallorca | 2021/08/30 08:00 | Martos Gornes, Sergio v Haerteis, Johannes | 1-6,1-6 |
| Challenger - Prague | 2021/08/24 10:15 | Haerteis, Johannes v Alex Rybakov | 4-6,2-6 |
| Luedenscheid, Germany | 2021/08/15 09:30 | Bogdan Bobrov v Haerteis, Johannes | 1-6,6-4,6-3 |
| Challenger Meerbusch | 2021/08/08 16:05 | Nedelko, Ivan v Haerteis, Johannes | 7-6,6-3 |
| Segovia, Spain | 2021/07/27 13:05 | Altug Celikbilek v Haerteis, Johannes | 6-3,6-4 |
| Challenger Pozoblanco | 2021/07/20 20:10 | Cem Ilkel v Haerteis, Johannes | 6-2,7-6 |
| Challenger Amersfoort | 2021/07/13 08:30 | Moraing, Mats v Haerteis, Johannes | 6-1,6-0 |
| US Open, New York, USA | 2021/08/25 16:15 | Lacko, Lukas v Holger Rune | 4-6,4-6 |
| Atlanta, USA | 2021/07/24 19:25 | Jason Jung v Lacko, Lukas | 6-4,6-7,6-3 |
| Challenger Cary | 2021/07/19 23:45 | Torpegaard, Mikael v Lacko, Lukas | 6-4,6-2 |
| Newport, USA | 2021/07/11 18:30 | Lacko, Lukas v Schnur, Brayden | 0-6,4-6 |
| Wimbledon, London, Great Britain | 2021/06/22 10:00 | Lacko, Lukas v Nikola Milojevic | 4-6,4-6 |
| ATP Halle | 2021/06/17 12:00 | Lloyd Harris v Lacko, Lukas | 6-3,7-6 |
| ATP Halle | 2021/06/15 09:00 | Pella, Guido v Lacko, Lukas | 7-6,5-7,1-6 |
| ATP Halle | 2021/06/13 10:40 | Lacko, Lukas v Joao Sousa | 4-6,6-4,6-3 |