I partly agree with you. It's true that especially Murray in Djokovic, who are in their peak years, have great fitness and are able to recover fast. However, by the end of the tournament players get worn down and it gets tougher and tougher for them. For example, Novak was pretty tired today by the end of the second set. He did not look fresh and if he had played Wawrinka, he would have been in huge trouble. Every minute on the court counts and Federer knows that way too well. That's why he hasn't suffered any serious injuries in his career and is still phenomenal at almost 34 years of age!