(Adds later matches, quotes)
By Larry Fine
NEW YORK, Aug 27 (Reuters) - Chinese doubles specialist Peng Shuai supplied a shocking start to the second round of the U.S. Open by upsetting fourth-seeded Agnieszka Radwanska of Poland 6-3 6-4 on a hot, humid Wednesday at Flushing Meadows.
The doubles world number one used piercing, two-fisted 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.
Radwanska, winner in the Montreal hardcourt run-up to the U.S. Open and a semi-finalist at this year's Australian Open, saved a match point in the ninth game to hold serve for 5-4 and fended off another in the next game before Peng's last winner.
The 28-year-old Peng, the French Open doubles champion and 2013 Wimbledon doubles winner who is still seeking her first career WTA Tour singles title, registered 28 winners in the 96-minute match at Louis Armstrong Stadium.
"I had two match points and didn't make it," Peng said in an oncourt interview about two backhand errors that prolonged the match.
"But I just said 'fight, fight' and it's an amazing time for me," added Peng, who has 16 career doubles wins and in February became the first Chinese player to become a world number one in tennis.
The Chinese, ranked 39th in singles, will play 28th seed Roberta Vinci in the third round. The Italian, also a renowned doubles player, advanced with a 2-6 6-4 6-1 win against Irina-Camelia Begu of Romania.
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 tiebreaker, wasted little time as temperatures soared.
"I started to be more aggressive and hit the ball," said second seed Halep. "I feel great now that I could win so fast today because it's so hot outside."
Halep, the French Open runner-up, will next meet either Shahar Peer of Israel or Croatian Mirjana Lucic-Baroni.
Johanna Larsson of Sweden 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.
Also advancing to the third round was 22nd seed Alize Cornet of France, a 6-3 6-3 winner over 37th-ranked Daniela Hantuchova of Slovakia.
On the men's side, sixth seed Tomas Berdych of the Czech Republic dismissed 2001 U.S. Open champion Lleyton Hewitt of Australia 6-3 6-4 6-3 in his opening-round match in Arthur Ashe Stadium.
Marin Cilic of Croatia, the 14th seed, and 19th-seeded Spaniard Feliciano Lopez advanced in abbreviated fashion.
Cilic was ahead 6-3 3-1 when Marcos Baghdatis of Cyprus retired with a leg injury, and Lopez was declared the winner at 1-1 in the fifth set when Croat Ivan Dodig was unable to continue.
Peng said she received some positive support from fans before the match against Radwanska.
"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."
The Chinese, who entered the contest 3-22 lifetime against top-five opponents, was masterful in the turning points of the match. Peng won three of five break points against Radwanska, who succeeded only once in seven opportunities to break.
"Every game was pretty tight and a lot of rallies," lamented a disappointed Radwanska. "So every game could go either way. But in the important moments I think she was just playing a little bit better."
Peng, with six career singles runner-up finishes, including this year in Shenzhen, will be aiming to improve on her best grand slam singles result having reached the fourth round four times, including at this year's Wimbledon.
"I still have a lot to learn," Peng said. "I will try to keep on fighting. I still have to working hard, keep going and fighting and improve my tennis." (Editing by Frank Pingue)