(Reuters) - Novak Djokovic's single-minded drive towards greatness makes it almost unthinkable that he would rest before owning every tennis record in the book, but some are questioning whether his hunger will survive the departure of his two main rivals.

The Serbian came within one win of a 21st Grand Slam title last week that would have moved him clear of a tie with Roger Federer and Rafa Nadal and also made him the only man in 52 years to claim all four major titles in the same year.