NEW YORK — Andy Murray sent an ominous warning to his US Open rivals when he breezed through his first match at Flushing Meadows on Wednesday before home hope Andy Roddick made an undignified exit in the last match of the day.

In the women's draw, former champions Kim Clijsters and Venus Williams dazzled their opponents and charmed the crowds after Victoria Azarenka had sent a shiver through the tennis world when she fainted on court.

Roddick was outplayed by unseeded Serb Janko Tipsarevic, but his 3-6, 7-5, 6-3, 7-6 (4) defeat was reduced to a footnote after the ninth seed's churlish rant with a line judge over a foot fault that had no bearing on the result.

"In hindsight, did I let it go too far? Yeah, probably," Roddick later said.

Much earlier in the day, it was a tumble on one of the outside courts that sent pulses racing at the year's last Grand Slam.

Azarenka, in a black dress and playing when temperatures were at their hottest, was rushed to hospital after collapsing in a heap at the baseline during her second-round match against Argentina's Gisela Dulko.

The Belarussian had succumbed to scorching heat at last year's Australian Open, and there were fears she had been a victim of the extreme temperatures that had forced US Open organizers to invoke their rarely used Extreme Weather Policy.

She later revealed she had been diagnosed with a mild concussion and her frightening crumble was a delayed reaction to a fall suffered earlier in the gym while warming up.

"I was checked by the medical team before I went on court and they were courtside for monitoring," said Azarenka, who was trailing 5-1 in the first set when she collapsed.

"I felt worse as the match went on, having a headache and feeling dizzy. I also started having trouble seeing and felt weak before I fell."

Murray showed why he is regarded as a serious contender to capture the men's title as he crushed Slovakia's Lukas Lacko 6-3, 6-2, 6-2.

The perennial hope of British tennis, Murray succeeded in escaping the worst of the roasting heat by racing to victory in under two hours.

"I managed to get through in straight sets so I'm pleased, but it was a lot tougher than the score suggests," the fourth-seeded Scot said.

"Everyone in my (supporters') box always tell me it's a lot tougher watching in the heat than playing, but I'm not so sure. It was brutal conditions."

Although the weather was slightly cooler than the previous day, it was still hot enough to leave players drenched in sweat and calling for ice packs to drape around their necks at the change of ends.

Beat the heat

New York's hottest summer in decades has spilled on to the courts at Flushing Meadows and contributed to a high dropout rate among the seeds.

Another eight seeds made early exits on Day 3, giving the main contenders extra incentive to quickly kill off their opponents.

Clijsters, who won the hearts of the crowds last year with her infant daughter watching from the stands, showed no mercy in her center court clash against Australian qualifier Sally Peers, who once pinned a photo of the Belgian on her bedroom wall.

"I'm just trying every match to grow so I'm ready when the opponents get tougher," Clij-sters said after thumping her inexperienced opponent 6-2, 6-1 in less than an hour.

Williams had no room for sentiment either as she romped to a comfortable 7-6 (3), 6-3 victory over Canada's Rebecca Marino, who gave the third seed a brief scare in the opening set before succumbing to the American's power.

Williams is only playing singles this year after her sister and regular doubles partner Serena pulled out with an injury and said she was loving the lighter workload.

"So far I feel good. I made a quick turnaround this summer to try to get ready for the Open," she said.

"I'm glad that I'm just in the singles. That way I have the opportunity to recover between rounds and to get ready to play the next one."

Williams has not won the US Open title since 2001 but she now represents the host nation's best hope of a champion after Roddick, the highest-ranked American male, bowed out.

Italy's French Open champion Francesca Schiavone and Australia's Samantha Stosur, the woman she beat in the Paris final, both advanced to the third round in straight sets, while the other most notable casualty in the men's draw was Wimbledon finalist Tomas Berdych.

The Czech stumbled at the first hurdle, outclassed 7-6 (3), 6-4, 6-4 by France's Michael Llodra, who is better known as a doubles specialist.

"I don't know whether I played well or not today," Berdych said. "I need to sit down with my coach who saw the match from the stands."