Leland de la Durantaye at Cabinet Magazine:
Federer exercised an animal fascination over the children, and myself, that was different from that of Roddick or Murray or the others. This may have had something to do with fame, but many were too young for that. Like most great athletes, like Jordan, Kobe, Zidane, Messi, lions, etc., Federer is more fluid than those around him. His body and being appear to be extremely relaxed until the moment of movement, which is fast, smooth and has something lethal about it. What makes him so unreadable a player (he is famously difficult to anticipate) is this very relaxation, for it is tension that opponents can learn to read, whereas relaxation is illegible. No one arrives so perfectly on time for his meeting with the ball as Federer, not only never too late, but never too early, so that all the momentum of his arrival flows into the shot with what appears to be almost effortless violence. Most professionals do this approximately, often with lots of fine-tuning little steps at the last minute (right before impact, Nadal, for instance, takes a series of hunched mini-steps). Federer does it so exactly that his strides are long, which is only possible thanks to the unearthly sense he obviously has of where his body is in relation to everything, always.
more here.