I think that Novak would be the favorite in a Clay match-up with Fed (I'd give Nole a 60-65% chance to win). But you would be less surprised if Nole lost to Almagro than Federer??
Novak is a different animal when he faces one of the top guys, Mugro could pull a Wawrinka and get to the finish line...if it does come to this, let's hope he can break for the match and doesn't have to serve it out.
As for Roger, we all saw what Novak did to him on clay last year.