In 2001 at the FINA World Championships in Fukuoka, Japan, Klaas-Erik was a member of the silver medal winning 4×100 meter freestyle relay team. Later, along with teammates Pieter van den Hoogenband, Johan Kenkhuis and Mitja Zastrow, Zwering won his first Olympic silver medal in the 4×100 meter freestyle relay in the 2004 Olympic Games in Athens, Greece. Afterwards he retired.