New York, Sep 2 (AP): Defending champion Kim Clijsters and two-time winner Venus Williams advanced to the third round at the U.S. Open in straight-sets wins that were overshadowed by a concussed Victoria Azarenka's collapse on the court. 

Azarenka, a 21-year-old on the rise in the WTA rankings, paused about a half-hour into her second-round match against Argentina's Gisela Dulko, then staggered, stumbled and collapsed to the court yesterday.

The 10th-seeded Azarenka rolled over to rest her head on her arm, and a trainer rushed over. She eventually was helped into a wheelchair, her yellow visor askew atop her head, then taken to a hospital, where tests showed she had a mild concussion.

As a record-breaking summer suffocates New York, the temperature on the courts at Flushing Meadows topped 100 F (38C). But tournament referee Brian Early said Azarenka's problem did "not seem to be primarily a heat-related illness."

Indeed, Azarenka herself later revealed she fell in the gym while warming up before the match, banging her head and arm in the gym.

"I was checked by the medical team before I went on court and they were courtside for monitoring. I felt worse as the match went on, having a headache and feeling dizzy," said Azarenka, who is from Belarus but lives part of the year in Arizona.

Azarenka began wobbling early against Dulko, taking extra time between points and wincing occasionally, clearly in distress. Azarenka said she "started having trouble seeing and felt weak."

It was by far the most stunning developing on Day 3 of a tournament that produced some surprising results on the scoreboard, including 18-year-old American qualifier Ryan Harrison's 6-3, 6-7 (4), 6-3, 6-4 victory over 15th-seeded Ivan Ljubicic, and unseeded Michael Llodra's 7-6 (3), 6-4, 6-4 upset of Wimbledon runner-up Tomas Berdych.

Winners included No. 4-seeded Andy Murray, the 2008 runner-up; No. 12 Mikhail Youzhny; No. 14 Nicolas Almagro; and No. 20 Sam Querrey, who beat NCAA singles champion Bradley Klahn 6-3, 4-6, 7-5, 6-4.

At night, Clijsters reached the third round by beating 201st-ranked qualifier Sally Peers of Australia 6-2, 6-1.Venus Williams, who counts the 2000 and 2001 U.S. Opens among her seven Grand Slam singles titles, struggled for a bit against 193rd-ranked qualifier Rebecca Marino of Canada before pulling out a 7-6 (3), 6-3 win.

Marino actually led 3-1 in the tiebreaker, before Williams took the next six straight points to steal the opening set.

"She started returning better, serving bombs. I think she definitely upped her level," Marino said. "I did notice at one point that she started to grunt pretty loudly. And it's like, 'Whoa, she's getting serious here."'

Harrison was the first U.S. male teen to beat a top-20 opponent at any Grand Slam tournament since a 19-year-old Roddick knocked off No. 11 Alex Corretja at the 2001 U.S. Open.

You don't have to go nearly as far back to find a female teen from the United States who pulled off that sort of upset, of course: Melanie Oudin was 17 a year ago when she reached the U.S. Open quarterfinals by beating two top-20 players.

Oudin's 2010 stay in New York was shorter: She lost in the second round Wednesday to No. 29 Alona Bondarenko 6-2, 7-5.

"Definitely disappointing," Oudin said. "I still have, hopefully, like, 10 more years in my career, hopefully 10 more U.S. Opens ahead of me. So I'll definitely be looking forward again to next year."

Among the seeded women bowing out was No. 21 Zheng Jie, overwhelmed 6-3, 6-0 by 2008 French Open champion Ana Ivanovic, who used to be ranked No. 1 but is now ranked No. 40.

No. 13 Marion Bartoli, No. 28 Alisa Kleybanova and No. 32 Tsvetana Pironkova also lost. Pironkova was ranked only 82nd in June, when she shocked Venus Williams in the quarterfinals, and they could have met in the third round in New York. But Pironkova bowed out in straight sets against qualifier Mandy Minella of Luxembourg, who gets to face Williams instead.

Azarenka is an up-and-comer on tour, part of a group of young players seen as potential future Grand Slam champions. She beat Maria Sharapova in the final of a hard-court tournament in California last month and pushed Serena Williams to three sets before losing in the Australian Open quarterfinals in January.

Against Williams in the fourth round of the 2009 Australian Open, Azarenka stopped in the second set, dizzy and in tears, and blamed a virus.

Her frightening exit Wednesday caught everyone's attention. Top-seeded Caroline Wozniacki wrote on Twitter: "Did anyone watch Vikas match?? I really hope she is ok!"
The match was halted with Azarenka trailing Dulko 5-1.

"It was terrible. It's not nice to see someone feeling bad, not nice to win a match this way. I hope she feels OK now," said Dulko, who walked around the net to check on the prone Azarenka. "I was worried for her."