The above three players have hit their summit, Novak can stay there for a while the other two have declined if only slightly because they have been overtaken by Novak.
Andy is still on the assent not yet reaching his summit. The question is will it be higher than Novak's???
The way Novak keeps punishing his body I can see burn-out beckoning in a year or two. Last year he was a physical wreck by the end of the USO (right shoulder problem, torn muscle in his back which he aggravated a week later playing DC) and was unable to play for the rest of the season, apart from a poor showing at the WTF. Also he's got a huge task in front of him defending all these ranking points, and he knows it, which is why he got so nervous at the AO. As for Rafa and Roger - Rafa's another whose body is wrecked whilst Roger certainly isn't the player he used to be. The one player who could possibly challenge Andy in the future is Del Potro, provided he can stay healthy and get back the form that won him the 2009 USO.