NEW YORK: Rain and lightning fears halted play at the US Open on Wednesday, stopping seven singles matches and delaying the start of matches for past champions Venus Williams and Juan Martin del Potro.

Officials said play would not resume before 2:30 p.m. local time (1830 GMT) in part because of threatening lightning in the area.

Williams, the 60th-ranked winner of the 2000 and 2001 US Opens, was set to face China's 56th-ranked Zheng Jie at Louis Armstrong Stadium.

Argentina's fourth-seeded Del Potro, the 2009 US Open winner, was preparing to meet Spain's Guillermo Garcia-Lopez at Arthur Ashe Stadium.

Earlier Wednesday, Agnieszka Radwanska and Li Na reached the US Open third round, as defending men's champion Andy Murray impatiently waited to start his New York campaign.

Radwanska, promoted to third seed after the injury-forced withdrawal of Maria Sharapova, defeated Spanish world No. 103 Maria-Teresa Torro-Flor, 6-0, 7-5 while Li, the Chinese fifth seed, beat 100th-ranked Swede Sofia Arvidsson 6-2, 6-2.

Radwanska, who has never got beyond the fourth round in New York, was on course for a quick-fire win over Torro-Flor, her second successive Spanish opponent, when she raced through the first set in just 21 minutes, losing just seven points.

But Torro-Flor, dressed in matching colours of blue and pink, made the Pole work for her win in the second set with three breaks of serve.

Radwanska, however weathered the unexpected storm, going through to a third-round clash against Russia's Anastasia Pavlyuchenkova, capitalising on her rival's 38 unforced errors and 10 double faults.

Li, playing in her 30th Grand Slam and a New York quarter-finalist in 2009, clinched her third career win over policeman's daughter Arvidsson, who has now not progressed beyond the second round in seven attempts.

The Chinese star will next face either British 30th seed Laura Robson, who knocked her out of the tournament in 2012, or France's Caroline Garcia.

"Both players have improved a lot in the last year," said Li. "I look forward to the chance of playing Laura."

Meanwhile, Murray begins the defense of his title almost 48 hours after second seed Rafael Nadal, viewed as the most likely contender to triumph this year, started his campaign.

"I'm playing my first round match at the us open at 7pm on a wednesday...," tweeted Murray in thinly-veiled frustration at a schedule which will see even first-round matches in the mixed doubles completed before he takes the court.

Federer, Djokovic cruise; Stosur upset

On Tuesday night, a teenage American girl with a squeaky voice and an incredible tale of survival stole the spotlight at the U.S. Open with a stunning upset win over the former champion, Sam Stosur.

While Roger Federer and Novak Djokovic and the rest of the sport's old order calmly went about their business, 17-year-old Victoria Duval became the toast of New York.

Making her second appearance in a grand slam event against the Australian who beat Serena Williams in the final just two years ago, the pint-sized Duval clawed and fought her way to a 5-7, 6-4, 6-4 first-round win.

It was a remarkable performance but made all the more astonishing because of her background.

When she was seven years old, Duval was taken hostage by robbers at her aunt's house in Haiti, a terrifying incident that convinced her parents, both doctors, to move back to the United States.

"It's not a good memory, so I try to forget as much as I could about it. I don't remember too much of it anymore, which is great," she told reporters.

In 2010, her father was buried alive in the Haiti earthquake. He survived by digging himself out but suffered serious injuries, including broken legs, broken ribs and a punctured lung.

Duval took to tennis instantly and has quickly risen through the American junior ranks but Tuesday's win was by far her biggest.

With her family watching from courtside and chants of "U-S-A!" echoing around the Louis Armstrong Stadium, Duval had to battle all the way to beat the vastly more-experienced Stosur.

"I think I'm very much of a child at heart ... (but) on the court, you have to be a warrior because that's just the sport we are in," she said.

Stosur paid tribute to Duval, saying she deserved the win, but said she had contributed to her own downfall with a whopping 56 unforced errors.

"I'm not going to be a sore loser and say she didn't do anything," said Stosur. "But, you know, I think I certainly helped her out there today, that's for sure."

Federer and Djokovic blasted their way into the second round with ruthless efficiency, crushing their hapless opponents in straight sets.

Defying the sceptics who had dismissed his chances of winning a sixth title in the Big Apple, Federer was in vintage form as he brushed past Slovenia's Grega Zemlja 6-3, 6-2, 7-5 in a delayed afternoon match at Arthur Ashe Stadium.

Djokovic then lit up Tuesday's night session with a dazzling combination of power and precision to defeat Lithuania's Ricardas Berankis 6-1, 6-2, 6-2.

The world No. 1 hardly broke sweat on a balmy New York night as he took the first step toward a possible fourth straight appearance in the final.

The 32-year-old Federer won the last of his five U.S. Open titles in 2008 but the Swiss master said he had lost none of his love of the game, ripping 35 winners in his win over Zemlja.

"I'm in a good spot right now. I want to enjoy it as long as it lasts," Federer said.

Twelve months after her agonizing defeat in the women's final, Victoria Azarenka made a triumphant return to Arthur Ashe Stadium, chalking up a rare double-bagel win by thrashing Germany's Dinah Pfizenmaier 6-0, 6-0 in just over an hour.

Italy's Sara Errani showed why she is looming as a real contender as she also handed out a 6-0, 6-0 thrashing to her Australian opponent Olivia Rogowska, who got into the draw as a 'lucky loser' when Japan's Ayumi Morita withdrew.