* Wawrinka made to work hard after easy opening sets
* Doubles specialist Peng upsets fourth seed Radwanska
* Sharapova, Halep and Venus Williams all advance
NEW YORK, Aug 28 (Reuters) - Second round action at the U.S. Open began with a shock and ended with some late-night drama as men's third seed Stan Wawrinka barked at fans before shouting for joy at his 6-3 6-4 3-6 7-6 (1) win over Thomaz Bellucci on Wednesday.
The day got off to an explosive start with Chinese doubles specialist Peng Shuai upsetting fourth-seeded Agnieszka Radwanska of Poland 6-3 6-4 and finished under the floodlights in the early morning hours with Wawrinka losing his cool and snapping at the rowdy crowd before escaping with a testy win.
After cruising through the opening two sets, Wawrinka had appeared set for a routine victory but the Swiss suddenly found himself in a titanic struggle with both the 95th-ranked Brazilian and some spectators.
In the third set, Bellucci charged 5-1 ahead to keep the match alive and then secured an early break in the fourth as a frustrated Wawrinka muttered to himself.
With the tension building and the score locked at 5-5 after the Swiss had broken back in the sixth game, Wawrinka's mumblings turned to open rage as he screamed at one heckler to "shut up man".
The Australian Open champion would, however, quickly regain his composure as the set moved to a tiebreak, which he dominated 7-1.
"I was trying to keep the focus you know when you drop your level against Bellucci, it is tough," Wawrinka said in a courtside interview. "He started to play better and it started to be a really tricky match.
"In the fourth set, I tried to focus more on my game. I was playing a little more aggressive at the end of the match and that is what made the difference."
By contrast, Peng, half of this year's French Open women's doubles winning pair, quietly went about her business, taking 96-minutes on a sun-baked Louis Armstrong Stadium court to claim the biggest scalp of the tournament so far.
Radwanska, winner of the Montreal hardcourt tune-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 to briefly delay the upset.
"I had two match points and didn't make it," said Peng, who has 16 career doubles wins and in February became the first Chinese player to be ranked as a world number one in tennis.
"But I just said 'fight, fight' and it's an amazing time for me."
Two former champions, fifth seed Maria Sharapova and 19th-seeded Venus Williams delighted their centre court audiences by flashing a little of their fashion style along with some on-court flair.
Williams, who claimed back-to-back U.S. Open titles from 2000, kept pace with younger sister Serena by taming 78th-ranked Timea Bacsinszky of Switzerland 6-1 6-4 to reach the third round for the first time in four years.
French Open champion Sharapova lost the first set to 95th-ranked Alexandra Dulgheru of Romania, but the five-times grand slam winner recovered to register a 4-6 6-3 6-2 victory and leave her fans smiling on a breezy Arthur Ashe Stadium court.
In another women's singles encounter, world number two Simona Halep of Romania beat the heat by hurrying past Jana Cepelova of Slovakia 6-2 6-1.
Halep, who overcame U.S. college champion Danielle Collins in the opening round after losing a first-set tie-break, wasted little time finding her top form as temperatures soared.
"I started to be more aggressive and hit the ball," the French Open runner-up said. "I feel great now that I could win so fast today because it's so hot outside."
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.
Johanna Larsson of Sweden, ranked 96th, sent another seed packing with a 5-7 6-4 6-2 upset of 21st-seeded American Sloane Stephens, a 2013 Australian Open semi-finalist.
On the men's side, sixth-seeded Czech Tomas Berdych dismissed 2001 champion Lleyton Hewitt of Australia 6-3 6-4 6-3 and seventh-seeded Bulgarian Grigor Dimitrov eased to a 6-2 7-6 6-2 victory over local wild card Ryan Harrison.
Other men's seeds advancing 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 cramp. (Editing by John O'Brien)