Entering his senior season, Grandjean competed in a sprint freestyle double at the 2008 Summer Olympics in Beijing. He achieved a FINA A-standard entry time of 49.09 (100 m freestyle) after finishing sixth from the European Championships in Eindhoven, Netherlands. In the 100 m freestyle, Grandjean placed nineteenth on the morning prelims. Swimming in heat seven, he blasted a new Belgian record of 48.82 in a Speedo techsuit, but narrowly missed the semifinal cut by only 0.27 of a second. Two days later, in the 50 m freestyle, Grandjean put up an astonishing swim with a fourth-place time and a Belgian record of 22.45 in heat nine, edging out Finland's Matti Rajakylä by a small fraction of a second. Grandjean failed to advance into the semifinals, as he placed twenty-eighth overall out of 97 swimmers in the prelims.