For me it`s easier with his backhand. Now looks like Fed always had this swing to be his main weakness, but till 2008 I never was scared every time he played backhand. But then... I think it had a lot to do with his loss to Nadal at Wimbledon. Not like before he hit the ball without EU and later couldn`t find a court, but something changed at least in my sensation. Cincy, Toronto, OG, even victory at US - backhand always gave a lot to think about. Hovewer Fed`s forehand remained to destroy opponents from any position.
But at some moment Fed lost it too. I mean he still has some great stretches with different kinds of shots from "open" racquet, but misses started to increase and the feeling about inevitable punishment for player who dared to hit the ball to Federer`s strong hand disappeared.
I was trying to remember my impressions, when I felt it happening the first time, but just can`t. Surely, the most important reason here is declining in speed. A bit worse footwork and so on. Such things always consist of many factors.
Winner of Wimbledon, 6 titles so far in 2012, and #1 in the world with a declined fh