Rafael Nadal pulls out of Paris Masters citing knee injury

The 31-year-old guaranteed that he would end the year atop the ATP rankings for the fourth time in his career after his second-round win over Hyeon Chung earlier in the week.

He clinched his place in the quarter-finals of the tournament with his 40th hardcourt win of the season - the first time he's reached that landmark since 2010 - after he progressed past Uruguay's world number 36, Pablo Cuevas.

But he has now taken himself out of the tournament in Paris due to a knee injury he hopes to recover from in time to end the season at the World Tour Finals in London.

The Spaniard has enjoyed a stunning season in which he has claimed two Grand Slam titles and grabbed and secured top spot again, having achieved the feat in 2008, 2010 and 2013.

"One year ago for sure I never dreamed about being world number one again," said Nadal.

"It is something that means lot to me. I'm very happy, but the season is not over."

Nadal bounced back from wrist problems last year to win his 15th and 16th Grand Slam titles at the French Open and US Open.

He sat out last week's event in Basel to rest his knees after a successful hard-court season but has clearly been unable to sustain his fitness in Paris.