Burnett began competing at an international level in 2000 at the 3 Nations Junior International. At the ASA National Championships, he won gold in the 50-meter backstroke in 2001 and 2002. On the advice of Pay, Burnett switched to swim freestyle in 2003 and began to excel. His television debut was in 2002 at the Manchester Commonwealth Games, where he competed in the 4×200-metre freestyle relay and won a bronze medal. The following Olympic Games saw Burnett finish 7th in the 200-metre freestyle, and he competed in the 4×200-metre freestyle relay team, which finished 4th, nearly grabbing bronze medals. The British trials for the Games had been a great success for Simon, in which he won a silver medal in 100-metre freestyle and a gold medal in the 200-metre freestyle, despite having broken his wrist in a cycling accident in Arizona two months previously. He was now emerging as an elite British swimmer. Swimming for the University of Arizona, Burnett competed in the American NCAAs and won gold in his first year in the 200-yard freestyle and in 2005. 2005 was probably his best year yet, returning to England as well for the ASA National Championships. He won three gold medals in the 50-metre, 100-metre, and 200-metre freestyle, all of which were new British records. In the United States, he also broke the 17-year-old record for the 200-yard freestyle set by Matt Biondi at 1:33.03; Burnett swam a 1:32.22.