In 1988, he joined the Sauber-Mercedes squad full-time, winning the German Supercup and finishing the World Sportscar Championship in second place, (behind Martin Brundle). before winning the WSC title in 1989 and 1990, on this occasion sharing the title with co-driver Mauro Baldi. His last season in WSC was 1991 during which he raced alongside Michael Schumacher with Sauber-Mercedes. He also won the "Classic Masters" title at the 1994 Race of Champions.
