Fonteyn competed for the Belgian swimming squad in the men's 200 m butterfly at the 2008 Summer Olympics in Beijing. Leading up to the Games, he placed fourth in the top-eight final at the European Championships in Eindhoven, Netherlands with an Olympic A-cut time of 1:56.86. Fonteyn blasted a new Belgian record of 1:56.65 to obtain the seventh spot and nineteenth overall in the last of six preliminary heats, narrowly missing out the top 16 semifinal by just sixth hundredths of a second (0.06) and trailing all-time Olympian and eventual world record holder Michael Phelps, who competed in the same race as Fonteyn, by nearly three seconds.