Frustration will retire Federer
Once he loses to all 3 players at all 4 slams in a repeated fashion, he will call it quits. Which will be around 2016.
That's already happened at AO
Murray has yet to (meet) beat him at RG.
Djokovic is yet to beat him at SW19
Murray/Nadal is yet to beat him at USO.
Q: How close are you and Djokovic?
Murray: Well he's in Dubai and I'm Doha, so around 450kms