I think Nadal has the best chance, Djo 2nd best, and Fed 3rd. Djo and Fed have been paired in the semis for the past few tourneys, which seriously hurts Fed's chances. Yes there is a day off, but only one day to recover from what is bound to be a grueling semi is not enough to be at 100%, and anything less than 100% against Nadal has no chance.Djo is younger and able to recover faster, so he has a decent chance if he gets by Fed.
It is possible that there is an upset, but much less likely than in the women's draw where upsets are routine and expected.