Wassen reached his highest doubles ranking on the ATP Tour on 10 September 2007 when he became World No.

The Heineken Open in Auckland, New Zealand, in 2006 and 2007 teaming up with Andrei Pavel and Jeff Coetzee respectively.

In 2009, he reached the finals of the 2009 Hall of Fame Tennis Championships in Newport, Rhode Island, partnering with Michael Kohlmann.

He is nicknamed "Da Professor" by good friend and doubles partner Dustin Brown.

He returned in doubles at 2013 Marburg Open, in partnership with Artem Sitak, losing in the first round against Vahid Mirzadeh and Denis Zivkovic