Nadal is a big threat to Hewitt- I was impressed with his fight during the 4th and 5th sets against Youzhny. Make no mistake- he doesn't have a big serve, but his forehand is both powerful and has some serious "workage" on it, his backhand is solid, and he moves like a cat.
I think Roddick has to be the favorite in the bottom half, even if Hewitt was in better form during the 2nd half of last year. This surface has been good to Roddick (SF in 2003, and would've been finalist if not for fatigue/wrist injury, and close 2004 QF loss to Safin), and his high bouncing serves could give the more diminuitive Hewitt trouble. Hewitt has never played very well here, with some strange losses, and I think Roddick has a slight edge over him on the Rebound Ace surface.
As for the top half, if Safin can't make the semis now, he should basically quit tennis. Having to beat Rochus and then Johansson/Hrbaty in the quarters (although he has lost to both Hrbaty and Johansson here before) should be a piece of cake, since he seems to be playing very well now. I know Andre looked really good against Dent, but I just do not see Roger losing to him. At the US Open, Andre had a lot going for him: The surface (Roger never played well at the Open), the crowd (obviously), and some awfully windy conditions (which Agassi has always said he likes), and Fed still prevailed.