well the way they grip the racket is part of this. I think (could be wrong as not an expert on this and happy to be corrected) that Djoko plays a full on western forehand. So, that's a high bouncing, rallying type player. Probably super fit. He certainly looks it. Federer I think plays one notch lower, a semi western, gives him more versatility but probably less energy game.Nadal he's a curious one. I think it's semi-western but not sure. Perhaps someone could tell me. He's generating the top spin and comfortable with the slice so full on western unlikely, but he also generates pace. But it's not flat so can't be anything like full western. anyway of these 3...I'd say Djoko or Nadal. Murray a year ago was in the equation as was and probably still is Ferrer who is simply a never say die beast.
The point I'm making although probably poorly is that the way you hold the racket probably has an influence on your fitness. Or rather if you hold your racket in a certain way...particularly western grip in the modern game you are more likely to be super fit to make it successful. not the only variable obviously but one that probably points to the sluggers in terms of fitness. Federer for me is less fit (but has more different types of shots). Not that he's unfit. He's probably the greatest in fact definitely......although Nadal may say something about that...who knows.