Everyone here is so busy talking about the stretch shortening cycle, esr/isr, and post modern 90mph groundstokes but I guess it's no fun to talk about the boring stuff that actually wins matches.
I divide shot tolerance into two categories, improving number one makes you a higher level player, improving number two helps you win at your level.
1: The quality of shot you can handle. If you are unable to return someone's serve or are overwhelmed by their rally ball, you are going to have a tough time winning.
So if two 4.0s can rally comfortably with each other then you introduce a 5.0 into the equation suddenly the 4.0 will start missing. Two 5.0s could do a cross court cooperative drill all day, but ask them to do it with Rafa Nadal and they would be lucky to get three balls back.
2: How many quality balls for your level you can hit in a point. Are you good for 4, 6, 8, 10+?
A very common scouting report is "yea, don't worry about it, he looks like a rockstar in the warm-up but once you start playing he can't hit 3 balls in the court.
When a veteran player plays a match the first thing he does is see if his opponent is going to miss. If the opponent is going to miss, then he goes no further, kicks on the cruise control and 6-3,6-2, shake hands, nice playing, great to meet you. If he isn't going to miss and the opponent isn't going to miss, then he actually has to play tennis.
J