Originally Posted by davey
It's clear from photos that Fed doesn't use the retail nSix-one. I noticed the shorter grip, similar to the PS 85, and am not surprised at the first post's description of the smaller head size. As far as the weight and string tension, Fed's own site says he varies these depending on playing conditions.
I saw at the US open that Fed is using the K-factor 90. The unique head shape of K-factor 90 is hard to miss. and also sound from the racket is also a metallic sound which denotes K-factor rackets.