His run included a win over the second seed and 2020 Australian Open - Boys' singles finalist Arthur Cazaux.

[5] In 2022, den Ouden won five ITF tournaments and his ranking climbed inside the top 500 for the first time, before being sidelined by injury.

[7] In November 2023, den Ouden defeated Arthur Géa to win an ITF Futures hardcourt tournament in Heraklion.

[8] He was given a wildcard into qualifying for the 2024 ABN AMRO Open in Rotterdam in February 2024, where he lost to alternate Pablo Llamas Ruiz.

[9] In September 2024, den Ouden won his maiden title at the 2024 Dobrich Challenger II in Bulgaria, where he faced compatriot Jelle Sels, defeating him in straight sets.