Novak was close to the net while Rafa was approaching to take a backhand shot, which could have easily been sent down Djokovic's line, not up his nasal cavity.
Novak did position his racket down and toward his left in anticipation of the down-the-line shot, making him a less protected target.
We rarely see such shots made by players at this level. Nah, no way.
Even at the upper-middle club level rarely do you see a player put a shot at another's grill, but it does happen.
Heck, I even hit my ex-wife between points one time and I can absolutely, positively, tell you it wasn't intentional. Check it out at http://itoaduso.blogspot.com/2013/08/InTheDogHouse.html.
Only these two guys really know. Or, at least one of them does.