If he plays like he did Haas, he's got a shot. He has just finished a five-setter though, which isn't ideal.
That said, he won after losing a set to Haas, he came back from two sets down this last game... It might show him that he can get back on top after getting a beating, and that's knowledge he'll need against Nadal.
From what I gathered from commentary, Murray has changed quite a bit. He's in control more. Compare it to Safin: when he's in control, he's nigh on unbeatable, when he lets people get to him, he helps them beat him. If Murray can keep a clear head, who knows what he can come up with. Tennis has seen crazier things happen.