* Peng upsets Radwanksa
* Sharapova survives in sweltering heat (Adds later matches, quotes)
By Larry Fine
NEW YORK, Aug 27 (Reuters) - The U.S. Open heated up on Wednesday with China's Peng Shuai upsetting fourth-seeded Agnieszka Radwanska and fifth seed Maria Sharapova averting an early exit on a sweltering day in Flushing Meadows.
French Open champion Sharapova lost the first set to 95th-ranked Alexandra Dulgheru of Romania, but the five-times grand slam winner bounced back to reach the third round with a 4-6 6-3 6-2 victory in warm breezes at Arthur Ashe Stadium.
Radwanska was not so fortunate as she fell 6-3 6-4 to world number one doubles player Peng, who supplied a shock start to second-round play in Louis Armstrong Stadium.
Men's first-round play was completed with sixth-seeded Czech Tomas Berdych dismissing 2001 champion Lleyton Hewitt of Australia 6-3 6-4 6-3 and seventh seed Bulgarian Grigor Dimitrov a 6-2 7-6 6-2 victor over local wild card Ryan Harrison.
Other men's seeds reaching the second round included Ernests Gulbis of Latvia (11), Marin Cilic of Croatia (14), South African Kevin Anderson (18) and Feliciano Lopez of Spain (19).
Cilic and Lopez advanced in abbreviated fashion. Cilic was leading 6-3 3-1 when Marcos Baghdatis of Cyprus retired with an ankle injury, while Lopez was at 1-1 in the fifth set when Croat Ivan Dodig could no longer continue due to cramping.
World number two Simona Halep of Romania beat the heat by hurrying past Jana Cepelova of Slovakia 6-2 6-1 to reach the third round.
Halep, who overcame U.S. college champion Danielle Collins in the opening round after losing a first-set tie-break, wasted little time as temperatures soared.
"I started to be more aggressive and hit the ball," said second seed Halep, the French Open runner-up. "I feel great now that I could win so fast today because it's so hot outside."
Johanna Larsson of Sweden, ranked 96th, sent another seed to the exits with a 5-7 6-4 6-2 upset of 21st-seeded American Sloane Stephens, a 2013 Australian Open semi-finalist.
Peng used piercing, two-handed groundstrokes to put Radwanska on the defensive and claimed the biggest scalp of the tournament so far on her third match point with a backhand crosscourt winner that brushed the corner lines.
A winner in the Montreal hardcourt run-up event to the U.S. Open and semi-finalist at this year's Australian Open, Radwanska was outplayed on critical points.
Peng won three of five break points against Radwanska, who succeeded only once in seven break opportunities.
"Every game was pretty tight and a lot of rallies," lamented Radwanska. "Every game could go either way. But in the important moments, I think she was just playing a little bit better."
Sharapova looked in trouble after making a massive 22 unforced errors in losing the first set against Dulgheru, who had risen to a high ranking of 26 in 2011 before knee injuries set her back.
But the 2006 champion recovered.
Sharapova broke in the sixth game of the second set to lead 4-2 and served out to claim the second set and start a run of five games in a row that took her to 4-0 in the third set and victory in the two hour 38 minute struggle.
It was a familiar formula for Sharapova, who leads the tour with 17 three-set victories this year.
"We started under the sun and finished under the lights. That's what you call the U.S. Open, I guess," said Sharapova.
"I'm very happy I was able to come back and come through and win this match."
As for being able to win the long matches, the Russian said: "A lot of hard work. You have to put in the hours and the miles, I guess."
The 28-year-old Peng, the French Open doubles champion, had 28 winners in the 96-minute upset of Radwanska.
"I had two match points and didn't make it," Peng said about two backhand errors that prolonged the match.
"But I just said 'fight, fight' and it's an amazing time for me," added the 39th-ranked Peng, who in February became the first Chinese player to become a world number one in tennis.
Peng, who has six career runner-up showings but no singles titles, said she received positive support from fans.
"A lot of fans (tell) me in the e-mail that I play her four times and the only time I win was in the U.S. Open four years ago," related Peng. "So maybe I saw the luck may be coming back today."
Others advancing included sixth seed Angelique Kerber of Germany, ninth-seeded Serbian Jelena Jankovic and 10th-seeded former world number one Caroline Wozniacki of Denmark. (Editing by Frank Pingue/Greg Stutchbury)