Injury forces Roddick out of Shanghai Masters

American Andy Roddick was once again forced to retire from an ATP tournament after injuring his knee in a second-round match at the Shanghai Masters.

Switzerland's Stanislas Wawrinka advanced with the score 3-4 in the first set after the World No 6 sustained the injury at 40-all in the eighth game, and immediately went to his chair.

Despite assistance from ATP trainer Michael Novotny and a doctor, the Wimbledon finalist - who recently complained that the top players' schedule was too taxing - was forced to give up and shake hands after 36 minutes on court.

The injury is another blow for the 27-year-old, who was forced to retire when he twisted an ankle in his semi-final against James Blake at Queen's Club in June.

The latest retirement is further setback in Roddick's disappointing season, which has seen him lose a third Wimbledon final to Roger Federer, exit the US Open in the third round and be defeated in the Beijing first round as defending champion.

"The pain is at the back of the (left) knee, and it's enough to make me stop a tennis match. My concern now is to find out what we are dealing with," said Roddick, who has yet to seal his place in the season-ending ATP Finals in London.

"I'll go back to the States and get it looked at. I suppose I'm lucky regarding qualifying (for the World Tour Finals) as I do have a bit of a cushion. But my main concern is dealing with this."

Three seeded players advanced to the third round - Russian No 6 Nikolay Davydenko beat compatriot Igor Kunitsyn 6-4 6-2, while Chilean 10th seed Fernando Gonzalez got a 6-3 6-4 victory over Brazil's Tomaz Bellucci.

Czech Radek Stepanek, seeded 13th, stopped German Andreas Beck 7-5 6-4.