That's just an extreme example though, where do you draw the line? Many times we see certain players who take their time and play long rallies (Massu, Nadal etc...) playing a first set over 60, maybe 70 minutes. In a set like this after an especially long rally these guys have very strong immidiate fatigue. The truth is there is not a way to have a black and white rule system about the time between points. It has to be up to the ump or else we are going to see a lot of ridiculous situations.
In other sports the athletes play when they are extremely worn out and physically struggling. You have timeouts in some sports but they are called based on the team rather than an individual, so often you see players really struggling and not running at full pace for long periods. Surely tennis players, especially considering it is a non-contact sport, will be able to fight through fatigue....and not stand still for 25-30 seconds and play with a towel. Also, tennis players do get to sit down quite a lot anyway.
Point I'm making is, there is no rule that says a tennis player has to be at his best physically for every point. It would only lift the fitness level of tennis, and make it a more physical game. But like I said, turn the shot clock off in the deciding set, and you get the highest quality of tennis in that set, and the drawn out suspense.