Exactly general. Just look at poor Kei Nishikori. He made a huge effort to beat Ferrer, and then had to come back a day later to beat Federer, and now he's out for I don't know how long. Though Kei has shown increases in strength and fitness this year, he has been prone to injury with his high octane game. I wish him well in his recovery.
These guys are not machines. It is not a team sport. They are only one man. There is nobody to replace them on the bench. If they get hurt, that's it. They are on the sidelines, fishing, playing poker, whatever, and watching the rest of the tennis world go by. Though there can be freak accidents, players at this level usually get hurt when they really have had to extend themselves to the fullest, not when they are playing easier matches. For the players who have a lot of matches in them, each injury might be the last.
Respectfully,
masterclass