Eren
Professional
USO 2011 to Cincy 2012 was remarkable for Federer he went 78-8 in that period. Hardly suprising he beat and troubled Novak in spite of his age. In this case you should look at his level as well.
I have already said often enough that that indeed was the turning point. He could compete up to and including the age of 31 with Djokovic. After that (i.e. 2012) it was game over.
By no means was I implying that 2011-2012 Fed was on amateur mode. He simply wasn't peak, but still very good. consistent enough to reach SF in Slams and had high peaks in certain matches only (could not perfom consistently on a high level during an entire tournament even).
When he was on, he was hard to beat on grass/hard in Slams but he showed kinks in his armour. Lost to Tsonga at Wimbledon after leading two sets to zero. Held MPs against Djokovic on his own serve and still lost at USO. Lost to Berdych at USO the following year. Reached only two Slam finals winning one. For Federer's standards that's pretty much a mug period.