Yes at this point in time:
Nadal on clay >>>>> Djokovic on hard courts
Nadal on grass >>>>> Djokovic on grass
Nadal on hard courts >>>>> Djokovic on clay
Not right now probably, but in overall career terms that is where it stands now. Djokovic has a long way to go to even reach Nadal's level. Heck before 2011 Nadal was much more accomplished and overall better than Djokovic on hard courts even, with Djokovic probably not passing him until the end of 2011. As for Nadal needing to prove he can beat Djokovic off clay, he has already done it many times in big matches, U.S Open final, Olympic semis, Wimbledon semis, so regardless what happens from here he doesnt have to prove a thing vs Djokovic. It is Djokovic who took until he was 24 and had shared over 4 years in the Worlds top 3 with Nadal before he could beat him a single big match anywhere, even on hard courts, lol! Djokovic is the only one with one thing to prove, that he can beat Nadal at RG, something that Federer failed to do, but I think Djokovic will succeed where Federer always failed at some point.