I don't know about that, I think he uses his flexability to his advantage more than hard gym work and sprinting etc, so he doesn't have to work as hard, Rafa has to work harder, and Murray too is in a similar situation, them and Ferrer I'd say are the fittest.
Novak's 6 hour grindfest at the Australian says otherwise. That was freakish the endurance he showed in that match. I hear what your saying about him using less energy than Nadal. But 6 hours of tennis is 6 hours of tennis. Novak is a machine.
I do agree Ferrer is very fit as well, but he can't last that long anymore at 30. Ferrer at 25, absolutely. Not now though.
Less impressed with Murray, but as a fan I'm pretty critical of him because I think he's an underachiever and it annoys me.