After the Olympic Games, Karstens started a successful professional career, where he won six stages in the
Tour de France, 14 stages in the
Vuelta a España, 1 stage in the
Giro d'Italia, and other races such as
Paris–Tours and
GP Fourmies. He became Dutch national road race champion in 1966. In the
1974 Tour de France, Karstens finished second in the fourth stage. Afterwards, he forgot to take the doping tests. The tour organisation set him back to the last place of the stage results, and gave him 10 minutes penalty time in the overall classification, which made him lose his third place. One day later, the jury took the penalty time back. Thanks to 5 seconds of bonification that Karstens won during intermediate sprints, he took over the
yellow jersey from
Eddy Merckx. He wore yellow for one stage before losing it to
Patrick Sercu after stage 6A, but he reclaimed the overall lead after stage 6B to spend another stage in Yellow before Merckx took over the lead for the remainder of the Tour. Karstens died of complications from a stroke on 8 October 2022, at the age of 80. ==Major results==