Most likely between Verdasco and Davydenko imo.
Verdasco hasn't been in good form/physical shape for a little while now and it is hard to look beyond R3 (Nalbandian) for him. Davydenko is also struggling, and Gasquet R2 is a little nasty. That being said, Gasquet unfortunately lacks the balls these days and tends to go out with a whimper.
Djokovic could be in a little bit of trouble...Troicki has managed to string together a few wins this week which might boost his confidence a little. The Nole of old would still be able to work him over, but these days nothing is guaranteed for him. If Troicki has an excellent serving day and tees off a little in his return games it could get very interesting.
Berdych could have a few problems with Llodra as well.
The other guys should be okay, although we don't yet know who Soderling will be playing (probably won't make a lot of difference).