Federer's got a shot, but the US Open is one of the most level playing fields out of the slams because there are so many excellent hard court players around these days. In other words, it's not just Djokovic that can cause him problems, no matter how brilliant a hard court player he is. Stan, Nishikori, Berdych and Cilic (I guess) can cause and have caused upsets in the past because they hit heavy and are rewarded on the fastest slam surface.
Rafa is playing Hamburg next week. On clay, but I guess he could do with practice, on any surface.