I think PHM can push Federer. With the crowd behind him, and nothing to lose, the pressure is on Federer. Federer will need to play better than today. PHM will need to play as well or a little better than today.
I know alot of people think PHM will choke against Fed., but I don't think so. He may lose, but I don't think he'll choke. He didn't choke against Nadal in the 3rd round in '06, though he lost. I expect a good match.