Think there was probably mental fatigue as well as some understandable physical fatigue. He said he felt he was rushing too much and making early errors ( when the stats apparently showed him winning more of the relatively longer rallies but Novak well on top in short "rallies" ) and he didn't really know why. He wasn't sure that he was too tired ( I assume meanibg physically tired ), but said he wasn't making some of these bad decisions earlier in the week when he had been pretty shatered from Valebcia.
I think a few days rest ( says he'll take a couple off anyway :) ) and get ready for the WTF. A bit fresher and he could do pretty well. Due a really good O2.
His record now vs Djokovic from 23 matches ( now his most against any player ) is won 8, lost 15. Though 2 - 2 in Slam finals.