Think Fed's biggest problem now, is not age but mentally.
Think about it, you are the GOAT, you are at the dawn of your legendary career, and then this year's Wimbledon, that was massive. And as a Djoko fan I do not say it in jest, I really mean it respectfully: Roger had maybe play one of the greatest matches ever, but it was just not meant to be, Novak peaked when it mattered most. And to lose a 3rd GS match like that against the same player, and on your holy ground, must get to you, he is human after all.
Now to make it even more difficult, these young guns are motivated, and they can upset any legend in 3 set matches, like what happened to Novak last year at Paris and finals. What must run through Roger's mind is:
After 18 years of great tennis, getting older, younger guys getting better: do I still want to fight it all. It is not that he cannot, he can any day, but the motivation and mental toughness not as it used to be?
I could be all wrong, and who knows, maybe he is back and win USO?
Top 5 Monday 6 
Think about it, you are the GOAT, you are at the dawn of your legendary career, and then this year's Wimbledon, that was massive. And as a Djoko fan I do not say it in jest, I really mean it respectfully: Roger had maybe play one of the greatest matches ever, but it was just not meant to be, Novak peaked when it mattered most. And to lose a 3rd GS match like that against the same player, and on your holy ground, must get to you, he is human after all.
Now to make it even more difficult, these young guns are motivated, and they can upset any legend in 3 set matches, like what happened to Novak last year at Paris and finals. What must run through Roger's mind is:
After 18 years of great tennis, getting older, younger guys getting better: do I still want to fight it all. It is not that he cannot, he can any day, but the motivation and mental toughness not as it used to be?
I could be all wrong, and who knows, maybe he is back and win USO?
But this is my opinion, looking at today's match.