Djokovic has a joke draw, as does Federer now that Monfils won the marathan match.
Neither Fed or Nadal had tough draws. Both should have cruised through to the semis. The fact that Nadal struggled against Isner on clay is beyond my understanding. Isner is not a great mover and clay is his worst surface.
Fed was playing more consistently because he faces players he has confidence against (such as Wawrinka, who is a MUCH better clay player than Isner or Ljubo).
Nadal has the tougher QF match in Soderling although Fed has a pretty tough match in Monfils as well.
Fed has the much tougher semi in Djokovic where Nadal has Murray.