| Challenger Stockton Qual | 10/01 18:40 | 46 | [266] Frederik Nielsen v Alejandro Gonzalez [485] | 6-4,6-1 | |
| Challenger Stockton Qual | 09/30 18:30 | 45 | Curtis Clarke v Frederik Nielsen | 1-6,4-6 | |
| Davis Cup | 09/17 10:00 | 28 | [313] Frederik Nielsen v Lloyd Harris [230] | 1-6,6-1,4-6,3-6 | |
| Davis Cup | 09/15 13:00 | 28 | [313] Frederik Nielsen v Nicolaas Scholtz [402] | 6-4,6-2,6-3 | |
| ITF France F18 | 09/04 12:55 | 25 | Frederik Nielsen v Romain Bauvy | 6-0,6-7,4-6 | |
| ITF France F18 Qual | 09/03 14:25 | 54 | Frederik Nielsen v Tommaso Metti | 6-2,6-4 | |
| ITF France F18 Qual | 09/02 13:20 | 54 | Frederik Nielsen v Louis Dussin | 6-1,6-2 | |
| ITF Portugal F18 | 08/29 16:00 | 25 | Wilfredo Gonzalez v Frederik Nielsen | 3-6,7-5,6-0 | |
| Challenger Vancouver Qual | 08/13 18:00 | - | [244] Frederik Nielsen v Takanyi Garanganga [444] | 4-6,7-5,3-6 | |
| Challenger Vancouver Qual | 08/12 19:05 | - | [288] Austin Krajicek v Frederik Nielsen [244] | 2-6,3-6 | |
| Challenger Aptos | 08/08 18:00 | 25 | [244] Frederik Nielsen v Tennys Sandgren [102] | 2-6,4-6 | |
| Challenger Aptos Qual | 08/06 18:35 | 54 | [247] Frederik Nielsen v Bradley Mousley [398] | 6-0,4-6,6-3 | |
| Challenger Aptos Qual | 08/05 23:00 | 46 | [209] Marc Polmans v Frederik Nielsen [247] | 6-3,1-6,4-6 | |
| Challenger Recanati | 07/05 19:00 | 26 | [121] Luca Vanni v Frederik Nielsen [226] | 6-2,2-6,6-2 | |
| Challenger Recanati | 07/03 13:30 | 25 | [226] Frederik Nielsen v Agustin Velotti [249] | 4-6,7-6,6-3 | |
| Challenger Recanati Qual | 07/02 12:00 | 54 | [441] Lorenzo Frigerio v Frederik Nielsen [245] | 4-6,4-6 | |
| Challenger Recanati Qual | 07/01 10:15 | 46 | Miki Jankovic v Frederik Nielsen | 7-6,2-6,4-6 | |
| ITF Portugal F9 | 06/29 16:15 | 26 | Emil Ruusuvuori v Frederik Nielsen | 6-3,4-6,6-3 | |
| ITF Portugal F9 | 06/27 15:20 | 25 | Benjamin Hannestad v Frederik Nielsen | 5-7,3-6 | |
| ITF Spain F18 | 06/22 18:15 | 27 | Raul Brancaccio v Frederik Nielsen | 7-5,2-6,7-6 | |
| ITF Spain F18 | 06/21 20:00 | 26 | Frederik Nielsen v Evan King | 6-4,6-3 | |
| ITF Spain F18 | 06/20 18:20 | 25 | Frederik Nielsen v Marek Gengel | 7-5,7-5 | |
| ITF Spain F17 | 06/15 18:45 | 26 | Frederik Nielsen v Alberto Barroso Campos | 6-4,2-6,1-6 | |
| ITF Spain F17 | 06/13 19:40 | 25 | Frederik Nielsen v Alastair Gray | 6-2,6-4 | |
| Challenger Busan Qual | 05/14 03:10 | 46 | [311] Austin Krajicek v Frederik Nielsen [207] | 6-3,6-2 | |
| Challenger Busan Qual | 05/13 03:45 | 45 | [444] Bradley Mousley v Frederik Nielsen [207] | 4-6,3-6 | |
| Challenger Seoul Qual | 05/06 01:00 | 45 | [193] Frederik Nielsen v Felix Auger Aliassime [338] | 1-6,4-6 | |
| Challenger Gimcheon Qual | 04/29 05:15 | 45 | [393] Lloyd Glasspool v Frederik Nielsen [170] | 6-7,6-2,6-2 | |
| Davis Cup | 04/09 10:00 | 27 | [130] Casper Ruud v Frederik Nielsen [152] | 4-6,2-6,3-6 | |
| Davis Cup | 04/07 16:15 | 27 | Viktor Durasovic v Frederik Nielsen | 2-6,6-3,2-6,6-7 |