The players he loses to tend to be big servers (that's when he loses at all). Obviously he lost to Isner recently and was taken to 5 sets against him in Davis Cup. He's lost 5 of his last 7 matches against Roddick. 4-5 Losing record against Tsonga, including a loss in 2011. 1-1 against Karlovic. We all saw him lose to Safin on grass in 2009. We saw Mardy Fish push him very hard in Cincinnati. etc etc. Now he doesn't lose to anyone at all very often, but when he does, it tends to be against big servers. A bit like Federer in 2006 with defensive players like Nadal, Murray, Canas etc. Big servers are the match up Djokovic likes the least.