Anything less than a 100% Nadal against the current Djokovic isn't going to get it done. Nadal needs to be 100% dialed in mentally and fully capable of going to the brink physically. Djokovic most likely is going to be making an all-out assault on clay this year and going for the Grand Slam. If Djokovic is playing on clay in the kind of form as what he showed at the AO and in 2011, Nadal will really have his work cut out for him.
This could be one of the best clay swings ever IMO. Del Potro just won Rotterdam and is also getting his confidence and form together. Murray and Federer are also massive potential scares across the way.