Born in
Rijswijk, Knopper began his career with
Feyenoord but, still in his youth, moved to rivals
Ajax, making his official debut in November 1997 against
Sparta Rotterdam. In 1999, he was voted by club supporters as
Most Talented Player, receiving the
Marco van Basten Trophy;
the following year he scored 15
Eredivisie goals as a midfielder, but his team could only finish fifth. A regular under
Jan Wouters, Knopper saw his playing time reduce when
Co Adriaanse took over as head coach in 2000. He was sidelined with injury for almost the entire
season, playing only eight matches that year and appearing slightly more in the following, which would be his last. In May 2002, Knopper had a trial with
Premier League side
Sunderland and played in that predicament in the
testimonial match for
Niall Quinn with the
Republic of Ireland on the 14th. From 2002 to 2004, he went out on two loans, first with Greece's
Aris and then with
Heerenveen, slightly reviving his career with the latter; upon his Ajax release in summer 2004, he spent two additional campaigns with
Vitesse, appearing regularly. After some problems with Vitesse manager
Aad de Mos, Knopper was forced to leave and joined
ADO Den Haag. In July 2007 a three-year contract was agreed, with the player netting again in double figures (ten) in
2008–09, crucial to help in a narrow escape from relegation; in the 2010 summer, aged 33, he was released. In January 2011, Knopper moved to
Liga Primer Indonesia club
PSM Makassar, leaving after a couple of months and signing with lowly
Haaglandia back in his country, where he ended his career. ==Managerial career==