NEW YORK - Venus Williams struggled with her serve. She kept tugging at her dress. Just as big a nuisance was her opponent, Shahar Peer.

On a day when No. 2 Kim Clijsters cruised to her fourth-round win at the U.S. Open, third-seeded Williams never looked quite comfortable in her 7-6 (3), 6-3 victory over 16th-seeded Peer on the second straight windy day in Arthur Ashe Stadium.

Williams got only 48 percent of her first serves in. She faced six break points and lost three. She squandered five chances to wrap up the first set in a 22-point 12th game. As for the dress — a red "daytime" version of the black, sequined number she wore two nights previously — well, she spent much of the match tugging at it to keep it at barely high-thigh level.

It was all part of a strange-feeling victory in which the two-time champion looked more like someone who was trying to find her form — which she is after missing most of August with an injured left kneecap — than someone breezing her way through the draw.

Her next match is a quarterfinal against No. 6 Francesca Schiavone, who had few problems in a 6-3, 6-0 win over 20th-seeded Anastasia Pavlyuchenkova.

Serving intelligently and handling Williams' power, Peer was surprisingly game, even though she fell to 0-6 lifetime against Williams and has yet to win a set. Trailing 6-5 and serving to stay in the first set, Peer staved off five set points before finally winning a game that took more than 12 minutes.

But Williams overpowered her in the tiebreaker to wrap up a first set that took 1 hour, 8 minutes.

Clijsters put a quick end to Ana Ivanovic's run, winning her 18th straight match at Flushing Meadows with a 6-2, 6-1 wipeout of the former world No. 1.

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Ivanovic, who fell to as low as No. 65 after a couple of injury-plagued years, has gotten back to No. 40 and was trying to move higher. She won three matches at the U.S. Open and was getting her biggest test — and opportunity — against the defending champion.

It wasn't much of a contest.

After regaining an early break to pull within 3-2 in the first, Ivanovic got overpowered, losing seven straight games to turn the match into a rout. Clijsters, moving as well as anyone in the tournament, used heavy, deep groundstrokes to pressure Ivanovic into 28 unforced errors. Looking like the more comfortable player, Clijsters fought through the wind and took command.

"She's playing with a lot more confidence," Clijsters said, in describing her mindset. "I can stay with her in the beginning of those first few games where she was playing really good tennis, if I could just stay with her and kind of just, make her doubt once in a while."

Trailing 4-1 in the second set, Ivanovic served a game that went seven deuces, but double-faulted to end the game. Eight points later, the match was over. It lasted 59 minutes. Ivanovic said she was nervous through much of it.

"I was on the big stage again," she said. "Lots of emotions came back and I just felt a little slow and just a little bit out of it."

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Clijsters next faces the winner of a match scheduled for later Saturday between No. 5 Sam Stosur and No. 12 Elena Dementieva.

The Belgian won the U.S. Open in 2005, then was off the tour for 2½ years while she got married and had a baby. She returned to win it again in 2009 and is still waiting for her first challenge at this year's tournament.

Other than a 7-5 second set in her first-round match, Clijsters has not been tested as she heads into the second week. She has lost a total of 14 games in four matches in what is the first true defense of her U.S. Open title because she missed 2006.

"Coming here as a defending champion has been a new experience and something that kind of keeps it fresh," Clijsters said. "It's a nice experience. And tennis-wise, as well, I've felt that I've been improving every match."

Clijsters will next face Australia's Sam Stosur, who won the latest-finishing women's match in U.S. Open history, overcoming four match points before beating 2004 runner-up Elena Dementieva of Russia 6-3, 2-6, 7-6 (2) in the fourth round.

There were 15 service breaks in the 2-hour, 38-minute match that ended at about 1:35 a.m. Monday when Stosur converted her second match point.

"When everyone stays out here and cheers and supports us like that, it couldn't be any better," 2010 French Open runner-up Stosur said in an on-court interview in Arthur Ashe Stadium.

The previous record for a U.S. Open women's match came in 1987, when Gabriela Sabatini and Beverly Bowes finished at 1:30 a.m. The latest finish at the tournament is 2:26 a.m., for a match between Mats Wilander and Mikael Pernfors in 1993.

The fifth-seeded Stosur is the first Australian woman to reach the U.S. Open quarterfinals since Wendy Turnbull lost in that round in 1986.

Stosur made her Grand Slam final debut at this year's French Open, but she had never been past the second round at Flushing Meadows. Playing high-risk, high-reward tennis against the 12th-seeded Dementieva, Stosur produced far more winners, 35-19, but also more unforced errors, 58-38.

"I think we both played a great match. Went for it and gave it our best," Stosur said. "To have a match like that here is just fantastic."

Dementieva reached the 2004 finals at Flushing Meadows and the French Open. This U.S. Open represented her return to Grand Slam tennis after missing Wimbledon with a left calf injury; before that, she had played in 46 consecutive major tournaments.

Dementieva held her first match point at 1:03 a.m., serving at 5-3, 40-30. But the Russian ended a nine-stroke exchange by pushing a forehand wide. Stosur then earned two break points and converted the second when Dementieva missed another forehand.

That got Stosur within 5-4, but she double-faulted at 30-all to set up a second match point, which Dementieva wasted by sailing a backhand long. Two more match points came in that game, and Stosur saved both, managing to hold serve for 5-all.

Stosur broke for the seventh time to go ahead 6-5 when Dementieva missed a forehand wide. Given a chance to serve out the victory, Stosur didn't make things easy on herself, putting a backhand into the net to give Dementieva a break point.

When an 18-stroke exchange closed with Dementieva netting a backhand, they were at deuce. Stosur hit a service winner at 111 mph to earn her first match point, then let that opportunity escape with a backhand of her own into the net.

Stosur controlled the deciding tiebreaker, though, taking the first three points and last three.

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