He won two Dutch National Time Trial Championships and represented his country at the 2004 Summer Olympics held in Athens, Greece.

In his first season as a professional he won Grote Prijs Stad Zottegem and stages of Critérium International and Tour de Pologne.

He also repeated his victory in the Dutch National Time Trial Championships and rode the Giro d'Italia.

The 2008 season got off to a promising start, with Dekker coming in 3rd place overall in both the Vuelta a Castilla y León and Tour of the Basque Country and achieving three top-ten finishes in the Ardennes classics.

[15] Although an early report in SportWereld said Dekker was on the verge of signing with Garmin-Chipotle,[16] team manager Jonathan Vaughters later denied this rumor.

[18] In 2009, Dekker finished a respectable 16th in the Tour of Switzerland, with a highlight of 3rd place in the second, 39 km long, individual time trial.

On 1 July 2009, it was announced that a re-test of an out-of-competition sample taken in December 2007, while Dekker was with Rabobank, was found to contain the banned substance EPO.

[19] Once his B-sample confirmed the EPO positive, Silence-Lotto fired Dekker, who admitted doping, apologizing and calling it "a mistake".

Garmin–Cervélo team manager Jonathan Vaughters challenged the two to beat the time he himself rode in 2001 with Jens Voigt.

[25] Dekker subsequently announced that he would focus on an attempt to break the world hour record in the spring of 2015 instead of finding a new team for the road cycling season.

[26] In a 2013 interview with Dutch daily newspaper NRC Handelsblad, Dekker stated that he started using performance-enhancing drugs when he joined Rabobank in 2005.

[27] Documents obtained during the Operación Puerto doping case proved that Dekker had been a customer of Eufemiano Fuentes, a well-known sports doctor who had assisted the Kelme team and many cycling professionals with blood doping; Spanish police found bags of Dekker's blood (Dekker was code-named "rider 24" and "Clasicómano Luigi"), and it turned out that he had had at least two transfusions in the spring of 2006, one four days before winning the Tirreno–Adriatico and another before riding the Tour of the Basque Country.