SHANGHAI: A bad season continued Tuesday for Andy Roddick as the American was forced to retire after injuring his knee in a second-round match at the Shanghai Masters.

Swiss Stan Wawrinka advanced 3-4 after Roddick -- next in the qualifying queue for a place in the eight-man World Tour Finals in London in November -- hurt his knee at 40-all in the eighth game and immediately went to his chair.

The fourth seed was joined by ATP trainer Michael Novotny and a doctor, with Roddick stretched out on his stomach for treatment before giving up and shaking hands after 36 minutes on court.

The injury could be a serious blow to the hopes of the former number one who lost a third Wimbledon final in July to Roger Federer, exited the US Open in the third round and last week lost in the Beijing first round as defending champion.

Roddick was also forced to retire before Wimbledon at Queen's Club when he twisted an ankle in his semi-final against James Blake.

Three other seeds in Shanghai advanced after byes at the penultimate Masters 1000 of the season.

Russian number six Nikolay Davydenko advanced over compatriot Igor Kunitsyn 6-4, 6-2. Davydenko has been on a run of form in the region, winning the Kuala Lumpur title and reaching last week's quarter-finals in Beijing.

Chile Fernando Gonzalez, number 10, beat Brazil's Tomaz Bellucci 6-3, 6-4 while 13th seed Radek Stepanek stopped German Andreas Beck 7-5, 6-4.

Roland Garros finalist Robin Soderling of Sweden beat Romanian Victor Hanescu 6-3, 6-4 and French 11th seed Gael Monfils moved into a clash with Hewitt after putting out compatriot Paul-Henri Mathieu 6-2, 6-2.

In first-round play, Lleyton Hewitt advanced 6-2, 6-4 into the second round of the Shanghai Masters over John Isner with the Australian's eyes firmly fixed three months into the future.

"This whole Asian swing is just about working on little areas of my game to try and make me as good a player for January as possible," said the former number one, who is keen to play again in his home Open in January.

Hewitt, competing in Shanghai for the first time since winning the season-ending Masters Cup in the city in 2002, held the normally powerful American Isner to a mere five aces, breaking three times.

The 28-year-old Australian has lifted his ranking from outside the Top 100 early in the year to its current 23rd, with further progress all part of a grand plan conceived after he underwent hip surgery just over a year ago.

"I haven't really focused a whole lot on the tournaments that I'm playing," Hewitt said.

"It's more a focus on little things I want to work on out there and improve on my game and try and become a better player for January -- then have a real crack next year."

Over the last years, Hewitt has barely registered on the ATP radar in the closing stages, playing his last complete autumn schedule in 2004.

"I've gotten better as the year has gone on. My hip's got a lot stronger and better as well, and that makes life a lot easier on the court," he said.