Shadow over Rafael Nadal's future as he pulls out of US Open

Doubts over Rafael Nadal's long-term future grew last night as the 26-year-old Spaniard withdrew from the forthcoming US Open because of the knee problems that continue to dog his career.

Nadal has not played since his second-round defeat to Lukas Rosol at Wimbledon nearly two months ago. He withdrew from the Olympics, from the Toronto Masters and from this week's Cincinnati Masters before confirming yesterday that he would not be fit to play in the year's final Grand Slam event in New York, which begins in 11 days' time.

The world No 3, who has been suffering from a recurrence of tendinitis in his knees, said: "I am very sad to announce that I am still not ready to play. I am sorry since I always found great crowds and support, but I have to continue with my recuperation and preparation to be ready to play in the right conditions."

Nadal's all-action style has always put pressure on his suspect knees and he finds hard courts particularly punishing. Clay and grass are more forgiving surfaces, which explains why his most profitable period of the year is usually between April and July.

Nevertheless, the Spaniard won the US Open in 2010 and was runner-up last year. After this year's tournament he will lose the 1,200 rankings points he would have been defending, which could give Andy Murray the chance to take his place at No 3. Murray, who beat Sam Querrey 6-2, 6-4 in the opening match of his title defence in Cincinnati last night to earn a third-round meeting with Jérémy Chardy, is only 695 points behind Nadal.

While questions will be asked about Nadal's future, past experience suggests it might be unwise to write him off. After suffering his only defeat in the French Open to Robin Soderling three years ago, he came back to win three Grand Slam titles in 2010 and reclaim his position at the top of the world rankings.

This year's early exit at Wimbledon ended Nadal's run of five successive appearances in Grand Slam finals, culminating in his seventh victory at the French Open. Having lost seven finals in a row to Novak Djokovic between last year's Indian Wells Masters and this year's Australian Open, he beat the Serb in three successive finals in the clay-court season.

Nadal's form in the first part of this year had already ensured his qualification for the Barclays ATP World Tour Finals in London, which bring together the year's eight most successful players. Roger Federer and Djokovic are the only others who have booked their places.

The Spaniard's absence will leave Federer, Djokovic and Murray as the favourites to win in New York. Federer reclaimed his position at the top of the world rankings with his victory at Wimbledon last month, Murray underlined his progress by reaching the Wimbledon final and winning the Olympic gold medal, while Djokovic won his first title for more than four months in Toronto last week.