Is 28 years old "post-prime"? I'm sorry, but that's nonsense. No reasonable person who saw the 2008 USO final and 2010 AO final could say that Federer wasn't peaking in those matches. He was phenomenal in both.
Well 2008 was probably where he started to decline from his unbelievably high peak as guys like Fish, Simon, Karlovic and even Blake beat him.
The first signs I saw that he wasn't the player of 04-07 were at the 09 USO final as Fed used to eat big hitters/ballbashers alive yet he was shanking Delpo's missiles.