|Azarenka is tended on court after collapsing during her match with Argentina's Gisela Dulko, left [AFP]
Former champion Andy Roddick crashed out of his home US Open to unseeded Serb Janko Tipsarevic as Andy Murray fired through to the second round in "brutal" conditions.
With temperatures soaring at Flushing Meadows in New York, Victoria Azarenka fainted on court and was taken to hospital during her second round match against Argentina's Gisela Dulko.
Kim Clijsters and Venus Williams, both two-time champions, fired to comprehensive wins in front of the packed Arthur Ashe Stadium stands.
Azarenka, in a black dress and playing when temperatures were at their hottest, was rushed to hospital after collapsing at the baseline.
The Belarussian had succumbed to scorching heat at last year's Australian Open and there were fears she had been a victim of the temperatures that had forced organisers to invoke their rarely used Extreme Weather Policy.
She later revealed that she had been diagnosed with a mild concussion and had a delayed reaction to a fall suffered in the gym while warming up.
"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," she said.
"Now I feel like I'm playing like a top 10 player, I have confidence that I can beat these players. That's huge for me"
Ana Ivanovic, former women's world no1
Top-seeded American Roddick stumbled to a bad-tempered second-round exit from the last grand slam of the year on Wednesday, losing 3-6 7-5 6-3 7-6 to Tipsarevic in the final match of the day.
The 44th-ranked Tipsarevic served magnificently and peppered all areas of the court with winners, despite having both ankles wrapped in bandages, and clinched the fourth-set tiebreaker 7-4 with a reflex volley at the net to eliminate the ninth seed.
The American kicked up a fuss in the third set when trailing 5-2 when he was told he had faulted with his right foot, when in fact he had touched the line with his left foot.
Roddick argued at length about the foot fault explanation offered by the lineswoman, although he later confirmed he was just trying to fire himself up.
The 26-year-old Tipsarevic will next meet France's Gael Monfils, who advanced past Russian Igor Andreev.
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.
"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 Scotsman said.
"It was brutal conditions."
Another seven seeds made early exits on day three, giving the main contenders with extra incentive to kill off their opponents quickly.
Clijsters showed no mercy in her match against Australian teenage qualifier Sally Peers, who grew up with a photo of the Belgian on her bedroom wall, thumping her inexperienced opponent 6-2 6-1 in less than an hour.
|Ivanovic says she is feeling like a better player than when she was world no1 [AFP]
Williams romped to a comfortable 7-6 6-3 victory over Canada's Rebecca Marino, who gave the third seed a brief scare in the opening set.
Williams is only playing singles this year after her sister and regular doubles partner Serena pulled out with an injury.
"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," she said.
A relaxed Ana Ivanovic stormed into the third round and said she finally feels like she is playing top-level tennis again.
The former world number one, who has dropped to 40th in the past year due to injuries and a loss of confidence, produced one of her best performances of late to crush 21st seed Zheng Jie of China 6-3 6-0 for a place in the last 32.
"I remember a couple years ago when I was here and I was saying, even though I'm number one I don't feel I'm playing as number one," Ivanovic said.
"But now I feel like I'm playing like a top 10 player, I have confidence that I can beat these players.
"That's huge for me."
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 most notable casualty was the men's Wimbledon finalist Tomas Berdych.
The Czech stumbled at the first hurdle, outclassed 7-6 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."