Yesterday, in the quarterfinal round of the Davis Cup, Novak Djokovic soldiered on with a sprained ankle to beat Sam Querrey in four sets, only the first two of which were competitive. Now, as Djokovic enters the most challenging part of the tennis season, taking on Rafael Nadal on the clay courts of Europe and then moving on quickly to the grass courts of Wimbledon, two slams held in less than one month, the word is that Djokovic might be seriously injured.