WIMBLEDON, England — Venus Williams advanced to the quarterfinals at Wimbledon when opponent Ana Ivanovic retired one game into the second set Monday.

Williams, seeking her sixth Wimbledon title, led 6-1, love-1 when Ivanovic called it quits.

Ivanovic took a 10-minute break during the first game of the second set to have her left thigh taped by a trainer. She returned for two more points, but after hitting a service winner to take the game, she began crying as she walked to her chair and told the umpire she was retiring.

Williams dominated from the start, taking a 5-0 lead before Ivanovic finally won a game 27 minutes into the match. Former No. 1 Ivanovic still hasn't reached a Grand Slam quarterfinal since winning the 2008 French Open.

American 17-year-old Melanie Oudin's surprising run ended when she lost to No. 11-seeded Agnieszka Radwanska, 6-4, 7-5.

Missed chances hurt Oudin, a qualifier from Marietta, Ga. ranked 124th. She flubbed an easy volley to lose the first set, and she was up a break in the second set.

Oudin, making her Wimbledon debut this year, pulled the biggest upset in the first week by beating former No. 1 Jelena Jankovic. Oudin was the youngest American to reach the women's fourth round at Wimbledon since Jennifer Capriati was a quarterfinalist in 1993.

Those reaching the gentlemen's round of 16 included six players age 27 or older, among them Tommy Haas, 31, and Radek Stepanek and Ivo Karlovic, both 30. The message: When it comes to the brief lawn tennis season, experience pays.

"Grass definitely takes some getting used to," said Andy Roddick, 26, who reached Week 2 for the fifth time in nine Wimbledons. "If you've played on it for years and years and years and years, I think the adjustment period will be probably a little bit quicker."

While returning the 130 mph serves common in the men's game requires a big adjustment on grass, novices sometimes thrive on the women's side. Among those reaching the ladies' fourth round were Oudin and 19-year-old Sabine Lisicki of Germany, who won a match on grass for the first time Tuesday.

"I just can't believe I'm in the fourth round," Lisicki said.

Two other teenagers made it — 19-year-old Victoria Azarenka of Belarus and 18-year-old Caroline Wozniacki of Denmark.

The youngest remaining player was also the most surprising. Oudin arrived in London with a ranking of 124 and an 0-2 record in Grand Slam matches, but she beat No. 29-seeded Sybille Bammer in the first round and No. 6 Jelena Jankovic in the third.

Oudin declined to grade her level of excitement.

"I'm going to answer that after the tournament's over," she said, "because it could get higher as the tournament goes on."

Oudin's so young her earliest Wimbledon memory is watching Venus and Serena Williams on TV. The sisters are still very much active — an all-Williams final Saturday is a distinct possibility.

Oudin's win over former No. 1 Jankovic was the most surprising result of the first week. Another upset: The new retractable roof on Centre Court was unused during the first half of the fortnight, causing snickers in the locker room.

"The common joke has been that they haven't had to use it yet," Roddick said. "All this money, and the weather has been nice."

There's still a long way to go, with a parade of talent Monday, when all 16 fourth-round matches were scheduled.

"It's a good ticket, I guess, if you're a tennis fan," Roddick said. "Even if you don't get on Centre, your grounds pass will do just fine."

The showcase matchup was five-time champion Roger Federer against Robin Soderling. Federer completed a career Grand Slam by beating Soderling in the French Open final three weeks ago.

On the women's side, Russians made up a quarter of the final 16. On the men's side, 13 countries remained represented, reflecting the global reach of the sport.