Stadium: Court 6 Round: 25 Ground: Clay
| ITF W15 Amarante | 2021/07/20 16:00 | Karen Marthiens v Ayla Aksu | 2-6,3-6 |
| ITF W60 Vitoria-Gasteiz | 2021/07/12 10:00 | Laetitia Pulchartova v Karen Marthiens | 7-5,6-4 |
| ITF W60 Vitoria-Gasteiz | 2021/07/11 09:20 | Lilly Elida Haseth v Karen Marthiens | 4-6,6-4,7-10 |
| ITF W25 Perigueux | 2021/06/22 08:00 | Karen Marthiens v Guiomar Maristany Zuleta De Reales | 3-6,0-6 |
| ITF W25 Madrid | 2021/06/13 12:00 | Andrea Burguete Beltran v Karen Marthiens | 6-0,4-6,10-7 |
| ITF W15 Heraklion | 2021/06/01 06:30 | Karen Marthiens v Antonia Schmidt | 1-6,3-6 |
| ITF W15 Heraklion | 2021/05/25 06:30 | Karen Marthiens v Eliz Maloney | 7-6,6-4 |
| ITF W15 Heraklion | 2021/05/24 08:00 | Karen Marthiens v Marija Semenistaja | 6-4,6-2 |
| ITF W25 Braunschweig | 2021/08/29 09:00 | Anna Klasen v Nastasja Schunk | 3-6,1-6 |
| ITF W25 Braunschweig | 2021/08/28 10:10 | Dalila Jakupovic v Anna Klasen | 6-3,0-6,2-6 |
| ITF W25 Braunschweig | 2021/08/27 16:50 | Anna Klasen v Tayisiya Morderger | 7-6,6-2 |
| ITF W25 Braunschweig | 2021/08/25 13:00 | Anna Klasen v Tamira Paszek | 6-2,6-3 |
| ITF W25 Koksijde | 2021/08/10 10:05 | Anna Klasen v Katharina Hobgarski | 6-7,4-6 |
| ITF W25 Koksijde | 2021/08/09 12:50 | Ekaterina Kazionova v Anna Klasen | 4-6,6-3,5-10 |
| ITF W25 Koksijde | 2021/08/08 08:00 | Anna Klasen v Lexie Stevens | 6-3,6-3 |
| ITF W60 Versmold | 2021/07/29 12:15 | Federica Di Sarra v Anna Klasen | 6-4,6-4 |