It is a lot more physical than it used to be, I think- and they play more. Years ago, too, the clay specialists would avoid Wimbledon, it was almost expected. Borg bucked that trend, and Chris Evert, by being great on clay and grass- which is hard. And why what Nadal has achieved is actually no mean feat, for someone who is not a natural on grass.
They were discussing it yesterday, I think, that whilst there is a break after the tour finals, it's not a 'rest' as in order to be ready for the next tournament, training can't stop. Most sports have a much bigger break. Like you have a football 'season', Tennis is pretty much 11 months solid.
And all he surfaces are very different and affect different bits of the body, so if knees are your achilles heel
so to speak, then grass is no great. Clay is kinder, because you can slide about.