Take a good look at these two rallying games
2009 us open semi final vs 2015 us open final
One of the reasons Federer lost power when he faced Djokovic in the Grand Slam is because he changed his racket from a small racket to a large racket
Of course, when you play against Nadal, the big racket will fit well
When I play with Djokovic, I really need the power of that forehand and backhand of a small racket.
And I think Federer might have won the 2015 us Open final if he played with a little racket more confidently.
That's why Federer kept losing to Djokovic in the majors, because he changed it from 2013 with a big racket.