I thought Federer would struggle against Wawrinka or at least against Monfils, but he is really playing his best tennis this year (Wilander said at least on clay he did). They have not faced each other on clay yet this year. I think Federer definitely has a chance although Djokovic is no doubt the favorite.
I agree with monstertruck in that he has to first of all serve well. And if he keeps the rallies mixed up and not one pace (which he has shown plenty in Roland Garros) he can come out on top. But if anything at all, again, he has to serve well and win enough free points on his service games. Once he can get the set to the later stages he will have a chance, his tie-break record at Roland Garros or any set that goes beyond 5-5 is excellent.