Take American baseball as an example. They play a 180 day schedule, with 19 days off, most of which are spent travelling. If you just grab a player in the clubhouse, on any given day, there's at least a 25% chance he won't be able to tell you what day of the week it is.
Sure, they battle, they fight, but it's a tough life.
Now, in international tennis, it's exacerbated. You aren't just talking about Arizona, then Colorado, then LA. You are talking all over the world.
It might seem great, and it is for a lot of players. But the grind gets to everyone. I don't care if you are staying at the Four Seasons in NY and the George V in Paris. It's still constant travel, constant jetlag, constant arriving, departing, getting settled just to leave a few days later.
It definitely takes its toll. And the season is longer now than it used to be. The grind is harder. And the players feel the effects, both on the court and off. That's why someone like Maria Sharapova isn't playing as many tournaments.
The season needs to be shortened, and the ranking system needs to be changed. Too many good, young players are getting ground up and thrown aside, due to both mental and physical stress.