Round: 44 Ground: Carpet indoor
| ITF Germany F14 | 2018/09/30 11:30 | Sieber, Marc v Nystroem, Patrick | 6-1,7-5 |
| Challenger Sibiu Qual | 2018/09/16 12:00 | Frunza, Nicolae v Sieber, Marc | 6-4,6-3 |
| Challenger Seville | 2018/09/01 16:00 | Sieber, Marc v Bernabe Zapata Miralles | 6-7,3-6 |
| Challenger Mallorca | 2018/08/26 11:30 | Federico Zeballos v Sieber, Marc | 3-6,6-1,6-3 |
| Challenger Meerbusch | 2018/08/14 12:30 | Sieber, Marc v Tatlot, Johan Sebastien | 6-7,6-4,4-6 |
| Meerbusch, Allemagne, Qualifications | 2018/08/13 09:00 | Tristan Lamasine v Sieber, Marc | 6-7,5-7 |
| Meerbusch, Allemagne, Qualifications | 2018/08/12 09:40 | Sieber, Marc v Adrian Oetzbach | 6-0,6-0 |
| Meerbusch, Allemagne, Qualifications | 2018/08/11 11:45 | Mats Rosenkranz v Sieber, Marc | 3-6,3-6 |
| ITF Switzerland F1 Qual | 2018/02/03 09:00 | Nicolas Parizzia v von Der Schulenburg, Henry | 4-6,5-7 |