Rafael Nadal has withdrawn from November's season-ending ATP Finals in London due to ankle injury. The withdrawal of Nadal, means Novak Djokovic is guaranteed to end the year top of the world rankings. 

American John Isner takes Nadal's place at the eight-player event in London. In a series of tweets,  Nadal said he needed an operation on his right ankle and is also yet to recover from the abdominal problem that kept him out of the Paris Masters last week.