Niels Desein mainly plays on the ATP Challenger Tour and the ITF Future Tour . So far he has celebrated 25 single and 24 double victories on the Future Tour. He won three titles on the Challenger Tour, including one singles in 2015 in Glasgow . On February 15, 2010, he broke through the top 200 in the world rankings for the first time in singles and his highest ranking was 154th in March 2015. In doubles, he broke through the top 200 for the first time on July 20, 2009 and his highest ranking was 182nd in August 2009.
He played his first game on the ATP World Tour in April 2013 at the Portugal Open in Oeiras , where he made it into the main field via the qualifying rounds . In the first lap he met Paolo Lorenzi , to whom he was defeated in three sets. In 2014 he was in the main draw of a Grand Slam tournament for the first time when he survived the qualifying rounds at the US Open . In the opening round he clearly lost to his compatriot David Goffin in three sets with 1: 6, 3: 6 and 3: 6.
In 2015 Desein played for the first time for the Belgian Davis Cup team , in which he was victorious in four sets against Switzerland in doubles with Ruben Bemelmans . Belgium won the match 3-2 and made it to the final, where they were defeated by Great Britain . This was not used in any other encounter.