Djokovic has developed the habit of offering up weak drop shots when he is down match point. Rafa put one away at Roland Garros and another at Queens. He also quits a higher percentage of matches than any other top ten player. Yesterday he found a new way to surrender, with two straight double faults to lose his second round encounter with a reborn Marat Safin.
These habits, along with his breathing problems (and the amount of talk he offers about his confidence in himself and the weaknesses he sees in Roger and others) appears to add up to an interesting case of mental fragility. Novak must talk himself into believing that he is the best, and when things go wrong for him it can kill that belief very quickly.
If you missed the match the best write up is from the BBC game by game blog.