You just opened the can to an endless, impossible to prove, debate. Often, when someone says something like this, the posts quickly go up and the debate becomes heated.
The fact that Lendl could post 130 wins without ever whining that he was tired, which I don't think he ever did, is hardly surprising given his training regimen, which was probably more stringent than Nadal's. I'd be curious to see a comparison, and am sure someone has done one...
That Gerulaitis could do it, with his binge drinking, cheese-burger eating lifestyle is something else, but then Vitas was never really a grinder.
It's not about the fitness. It's about the style. No matter how much Nadal trains or how good his physical condition is, he will invariably get tired because of his style. I don't think there's ever been a more accomplished defensive tennis player. Hopefully, he is planning his schedule in a way that allows him to maintain the highest level at the most important tournaments. If not, it's his own damned fault for playing the way he plays.
I, for one, am sick of people whining about their hero being tired. As a huge Nadal fan, I like to see him play well, but I also recognize that the way he plays has limitations, and exhaustion may very well be one of them.