Federer. Genius. Best Player of all time without any shadow of doubt.
But, I've seen that shot hit so many times on a tennis court (admittedly not in the semis of a slam). I could find you countless kids who can hit a through the legs 'hot dog'. He connected with it well, but I can remember him playing exactly the same shot against Marat Safin in the Aus Open final a few years ago, fluffing it and losing.
I'm more impressed with the rest of his game. He can do things other people can only dream of.