As an amateur, his most important results were second places at the youth world championships in Bergamo in 1987 and at the road race in the 1992 Summer Olympics.

In that Olympic road race, Dekker got away at 30 km before the finish, together with Fabio Casartelli and Dainis Ozols.

[3] Directly after the Olympic Games, he became professional, and rode his first race a few weeks later in the Tour de l'Avenir.

He won three stages in the 2000 Tour de France, although neither a sprinter nor a favourite for the overall win, and was voted most combative cyclist.

He had announced his retirement for the autumn of 2006, but he crashed heavily in the 2006 Tour de France and decided to stop.