To be honest, I don't know how much better athletes are really getting. I think in tennis, the training and sophistication has improved tremendously. In addition, we all keep hearing about what great athletes Nadal and Federer are and deservedly so. But I'm not sure these guys are better athletes than Sampras was. I think Sampras could move just as good as those guys, if not better. His serve, which just so happens to be the most important shot, was a lot better than anyone's today. In addition, it's not a stretch to think his ground game and backhand wouldn't be highly improved with poly strings and better racquet technology.
I personally think in a broader sense that athletes aren't any better than they were 20 years ago. Michael Jordan to me was more athletic than the basketball players today. Mike Tyson to me is a more explosive boxer than any we've seen today. Mike Powell long jumped farther than anyone can today. Sampras, in my opinion, was a better "athlete" than any player today....Djokovic, Nadal, whoever. I honestly believe Sampras would be better than all these guys.