Novak Djokovic, who won the US Open recently, is now favourite to unseat Rafael Nadal as world number one.

World number one Rafael Nadal said on Wednesday he would skip the Asian leg of the ATP tour to recover from the knee injury he picked up at the US Open, a decision that will boost Novak Djokovic's chances of ending the year top of the rankings.

Nadal was forced to retire during his semi-final against Argentine Juan Martin del Potro at Flushing Meadows with what he later said was tendinitis in his right knee, and will now also miss next month's tournaments in Beijing and Shanghai.

Djokovic, seeded sixth at the US Open, beat Del Potro in the final to claim his second Grand Slam title this year and rise to third in the rankings, behind Nadal and Roger Federer.

"I had to retire from the semi-finals of the US Open and this past Monday I was with doctors in Barcelona to review the situation with my knee," Nadal wrote on Twitter.

"While my knee problems are nothing new, I have decided together with my medical and technical team not to participate in the Asian tour to recover."

The Spaniard tops the world rankings with 8,760 points, 1,860 ahead of Federer and 2,315 more than in-form Djokovic.

With Federer cutting the number of tournaments he plays - the Swiss only took part in events in Shanghai, Basel and the ATP finals after last year's US Open - Djokovic is now favourite to unseat Nadal as world number one.