there's no way Federer loses to Ivanisevic, and the match will be short even if it's competitive. Roddick and Becker could make things interesting, but there's also a non-negligible chance Federer peaks and masterclasses them. Someone like Krajicek or Stich in place of Murray, not to mention Mac or older players, makes it significantly tougher. As it stands right now, Becker is the only one that stands a chance of really taking Fed by surprise if he's not on his game (Roddick can as well ala 2004 but likely will struggle to close the door), so if Federer peaks for that match, the first 4 rounds just aren't that hard.
Federer cleanly beat Djokovic in 2012, so yeah I think peak to peak I'm going with fairly clean.