Kessler debuted as a pro during the 2000 season after becoming German under 23 champion in 1999, a year in which he also took the bronze medal in the under 23 cycling world championships.

Despite the fact that he has only 3 wins in his palmarès, which includes two consecutive victories at the Gran Premio Miguel Induráin and LUK-Cup of 2003, he is seen as an outsider for the Ardennes classic races.

He had to abandon the 2004 Tour de France after a serious and spectacular fall caused him severe injuries, even though he managed to end the stage.

In January 2010, he had a collision with a cat while on a training ride in Mallorca, Spain, and was left in a critical condition with severe head injuries.

[3] Later named as a recipient of a blood transfusion at the University of Freiburg along with teammates Andreas Klöden and Patrick Sinkewitz during the 2006 Tour de France.