Klaassen qualified for the 2008 Summer Olympics in Beijing with the Dutch eights forming a team with Olaf van Andel, Rogier Blink, Meindert Klem, David Kuiper, Diederik Simon, Olivier Siegelaar, Mitchel Steenman and cox Peter Wiersum. Due to an injury Siegelaar was replaced by Reinder Lubbers during the tournament, where the Dutch finished in 4th place. He competed for the Dutch men's eight again at the 2012 Summer Olympics where they finished in 5th place.