Willemsen is the most successful rider in sidecarcross history, winning the title ten times, in 1999, 2003, 2004, 2005, 2006, 2007, 2008, 2010, 2011 and 2012. He has also won the Dutch national sidecarcross championship nine times, winning it last in 2008 and the Belgian championship twice.

For his success in motor sport he was made a member of the Order of Orange-Nassau in 2005. His home town Lochem also has named a street after him in 2005. He is the most successful World Championship sidecarcross rider and his ten titles place him seventh, as of 2011, in the all-time FIM World Champions list, with Giacomo Agostini with 15 titles at the top.

In 2007, he took part in the Dakar Rally, finishing tenth in the solo motorcycle class. To make up for the canceled Dakar Rally 2008, he took part in the Tuareq Rally in March/April 2008, winning the race.

Willemsen's sidecar experience started in 1981, when he and his brother Marcel, aged six and four, drove in a home-made Hond 50 cc side car, under the guidance of their father Berry, who had also been a sidecar racer. However, it took the brothers another ten years before they were allowed to take part in sidecar races.

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