He was selected as eighth-man for
Northern Transvaal whilst studying architecture at the
University of Pretoria in the late 1970s. He formed a loose-trio with
Thys Lourens and
Burger Geldenhuys and soon had two
Currie Cup final victories, in
1975 and
1977. In 1980 he moved to
Natal and was selected for the Springboks in 1981, captaining the team on his debut against
Ireland. Despite Natal's relegation to the Currie Cup B section at the end of 1981 he continued as Springbok captain until 1984. He played seven test matches for the Springboks, all as captain and also played in six tour matches, scoring two tries. Claassen is credited with reviving rugby at the
University of Natal, both as a player and later as a coach. In 1988 he coached the Durban campus of the
University of Natal to the
Moor Cup title.
Test history ==Springbok captain==