I am very sad to announce that I am not ready to play the US Open in NY. Thanks to my fans for their support and specially, the new yorkers.— Rafa Nadal (@RafaelNadal) August 15, 2012
Nadal missed the Olympics with tendinitis in his knee, which also caused him to pull out of the Rogers Cup and the Cincinnati Masters. It's likely that the same knee injury is the reason he's missing the US Open as well.
Hopefully this gives Nadal the rest that he needs to recover, because the game just isn't the same without him. Roger Federer, Novak Djokovic, Andy Murray, and others are putting together some outstanding tennis tournaments, but Nadal is definitely among the greatest ever, and it's a shame whenever he misses a tournament.