I have a feeling Nadal won't be so great on clay this year but will still reach the semi/final (depending on draw) and produce some mythical tennis against the grain of form in attempting to defy Djokovic... before losing.
I'd give this to Nadal in 5 sets. A bit like his 2013 campaign at Roland Garros actually. It would take a few select points to separate them, but I feel Nadal is just too clutch at Roland Garros not to pull through.