LONDON — Novak Djokovic knew all along that his game was good enough to win more Grand Slam titles. It was his mind that was the problem. And so after failing to convert a match point against Roger Federer in the fourth set of the Wimbledon final, and after losing five games in a row to get pushed to a fifth set,Djokovic left the court for a bathroom break so he could give himself a pep talk.