SHANGHAI — Andy Roddick was forced to retire with left knee pain while leading Stanislas Wawrinka 4-3 Tuesday in the second round of the Shanghai Masters.

Roddick, who limped into the post-match news conference, quit after wasting a break point.

"I just felt I pushed off, and then Michael (Novotny, ATP trainer) came on court and did some tests and advised that it probably wasn't worth the risk," Roddick said.

Roddick is vying for one of the final three spots at the season-ending ATP World Tour Finals in London next month. But the sixth-ranked Roddick says he's more concerned about taking care of his health than qualifying for the eight-man event.

"At this point, my concern is more along the lines of figuring out what we're dealing with," said Roddick, who has finished the last seven seasons ranked in the top 10.

The 22nd-ranked Wawrinka will play 13th-seeded Radek Stepanek of the Czech Republic in the third round.

"I hope he's going to be fine for the rest of the season," Wawrinka said of Roddick. "For sure, it's not the way you want it to go. But I'll take it and play the next match."

Lleyton Hewitt defeated John Isner of the United States 6-2, 6-4. Hewitt saved both break points he faced.

Hewitt said he's pleased with his progress since hip surgery in August 2008.

"This year was always going to be tough purely because I didn't exactly know how the hip was going to bounce back," Hewitt said. "I've got better as the year has gone on. My hip's got a lot stronger and better, and that makes life a lot easier on the court."

Sixth-seeded Nikolay Davydenko improved his perfect record against Igor Kunitsyn of Russia to 5-0 after a 6-4, 6-2 win. Tenth-seeded Fernando Gonzalez of Chile beat Thomaz Bellucci of Brazil 6-3, 6-4.

Fifteenth-seeded Tommy Haas beat Benjamin Becker 7-6 (5), 6-4 in an all-German match.

In an all-French match, 11th-seeded Gael Monfils defeated Paul-Henri Mathieu 6-2, 6-2.