|Wozniacki gor her challenge for a first grand slam off to a winning start against Chelsey Gullickson [AFP]
Rafael Nadal made a smooth start to his bid to capture the one grand slam that has eluded him when he wore down Russia's Teymuraz Gabashvili 7-6 7-6 6-3 in the first round of the US Open.
Tha Spanish world number one had to dig deep to see off the stubborn Gabashvili after three hours of intense shotmaking from both men at Arthur Ashe Stadium.
Neither player dropped serve in the opening two sets but Nadal's superior big-match experience came through when it mattered and he won both tiebreakers before Gabashvili finally cracked in the third set.
"I prefer to play one hour and 10 minutes. Everybody wants to win easier.," Nadal said.
"You play against (lower-ranked) players - they don't have much to lose, so they play aggressive."
After two days of few surprises, eight seeded players made early exits in the severe conditions.
The biggest casualty in the men's draw was Marcos Baghdatis of Cyprus, seeded 16th, while the highest women's seed to fall was China's Li Na, seeded eighth.
The exception was Denmark's Caroline Wozniacki, last year's runner-up and the top seed this time in the absence of world number one Serena Williams.
Wozniacki has been in devastating form in the past month, winning three tournaments to give herself the chance of a $1 million cash bonus, and made a flying start to her US Open campaign.
Her match did not begin until just before midnight local time because of the earlier backlog but she made up for lost time by thumping American wildcard Chelsey Gullickson 6-1 6-1 in just 61 minutes.
Novak Djokovic dispelled his reputation as a quitter after battling through five gruelling sets while Jelena Jankovic and Svetlana Kuznetsova were pushed to the brink of exhaustion as temperatures soared to 35C.
World number three Djokovic looked to be heading for an early exit when he fell two sets to one and a service break behind fellow Serb Viktor Troicki but somehow summoned up the strength to fight back and win 6-3 3-6 2-6 7-5 6-3.
His incredible performance came on a day when organisers invoked the tournament's rarely used extreme weather policy and fans fled the stands to seek refuge in the shade from the blazing sun.
For most of his match, Djokovic was soaked in sweat and gasping for air and playing an opponent showing no signs of weariness.
It was only in the fifth set, when the sun began to set and Troicki started to wilt, that relief finally came.
Djokovic made the US Open final in 2007 then won the Australian Open the following year but has been heavily criticised for retiring from matches at each of the four grand slams, including his defence of the Australian Open.
"I've been in those situations before, played a lot of long matches in very difficult conditions, feeling very exhausted," he told a news conference.
"You kind of start panicking a little bit when you don't feel great physically."
Jankovic, a finalist in New York in 2008, beat Romanian Simona Halep 6-4 4-6 7-5 after Kuznetsova, champion in 2004 and runner-up in 2007, had to work overtime to beat Japan's Kimiko Date Krumm 6-2 4-6 6-1.
Under the tournament's extreme heat policy, players are allowed to request a 10-minute break between the second and third sets if the mercury exceeds 30.1C before the start of the match.
Kuznetsova's match on the Grandstand court began before the policy was invoked, meaning the players could not have a break between sets, but they were given ice packs at the change of ends and were sheltered by umbrellas.
Maria Sharapova, the 2006 US Open champion, was also given a tough workout from Australia's Jarmila Groth in their match, played at dusk when the weather had cooled, before prevailing 4-6 6-3 6-1.
Sharapova's compatriot and Wimbledon finalist Vera Zvonareva had a much easier time against Slovakia's Zuzana Kucova on the Louis Armstrong stadium, winning 6-2 6-1.