Also, I look at Nadal's physique and have no doubt that he could have made it at wide receiver, point guard, third base, whatever. He's just a freak of nature.
EDIT: And I'll throw in a nomination for Mariano Rivera. Aside from being the most dominant reliever of all time, more than one source has been quoted saying that he could have been the best outfielder in the league had he chosen that path.