It's not all about America, you know.
Actually Nole is a huge star in Asia and that translates in a lot of €€€€.
So are Fed and Rafa
The companies looking for endorsement deals have already been treated to two very dominant tennis players, if they're looking for deals they'll either already have one of them or not be interested. This is demonstrated by the fact that although Rafa has been at the top or near it for almost the same length of time as Federer, the difference in their endorsement earnings is definitely greater than the difference in their career achievements, perhaps as Fed got there first. Plus Djokovic hasn't achieved half of what either of them have done yet, nor has he been at the top for very long by comparison. Fed and Rafa have proven themselves as sound investments, Djokovic hasn't yet.
Plus there's that small matter of global economic stagnation. Not too many companies will want to be spending much on endorsement deals.