as said before, sampras was definitely a great athlete. in fact, i think if he had been a track and field olympian, he would be a hurdler. he jumped quite well aside from running pretty fast.
as athletic as federer, nadal and djokovic are, i still think sampras is a slightly better pure athlete. i think nadal is stronger and has great footwork technique for slow courts, so he can get to lots of shots and bat them back with pace. djokovic is more flexible so he can run down lots of balls and use his smooth stroking technique from the waist up to send back quality shots, even if his lower body seems all over the place. federer might be a more instinctive mover on the court, with slightly better anticipation and coordination. but i think that in a pure sprint, sampras still edges all of them slightly. sampras doesn't need to rely too much on pure physical strength or flexibility to get to lots of balls.