I somewhat agree with those saying Djokovic moves a lot better here and is more fluent. Murray does plod a little more with his feet whilst Novaks flexibility and dexterity makes him recover his position better.
However I think it was pretty evident that it was Murray's forehand that let him down in the 4th. If that can be ascribed to exhaustion, tightness or technique is hard to say. But the problem is that Murray hits it so flat, that he nets a lot more than the other top players. Pulling those shots up from the ground up in the court is tough for Murray. Much like Berdych in that respect. I believe there is room for him improving his forehand technique, adding a little more topspin on some of those shots, changing grip when needed. Obviously he has been working with his FH throughout his career and it may just be what it is - not as good a natural shot as the other top guys and the first to break down when going the distance.
Let's see if Novak can finally get that elusive RG this year.