I agree with Federer here. You have to beat 5 of the top 8 players in the world, and in best of 3 so it becomes more about tennis skill than endurance. It's the most difficult tournament to win, definitely.
The fact you can lose a match and still progress is ridiculous. With the exception of the unique
Novak Djokovic, most players are exhausted after a long season and don't/can't play to their maximum. It's just a glorified exhibition that gives the ATP one last chance to fleece the punters before the holidays.
The Olympics laugh at how shit the WTF is.