Doornbos's first sport was tennis. His interest in motorsport grew after he was invited to the 1998 Belgian Grand Prix as a guest of WilliamsF1. It was 1997 Formula One Champion Jacques Villeneuve who suggested that he should go for a series such as Formula Ford, as he was too big and too old to race karts. He gave up tennis and focused on racing, joining the JR racing team for the 1999 Opel Lotus UK Winter series. He had a successful season, finishing second in the championship after taking four pole positions, four fastest laps and four wins. In 2000, he competed in the Formula Ford Zetec Benelux series, finishing second in the Belgian championship and fifth overall with one pole, three fastest laps and six podiums.