NEW YORK, Aug. 30 -- Top seed Novak Djokovic enjoyed a comfortable walk through into the second round of U.S. Open on Tuesday as his opponent retired trailing 6-0, 5-1, while French Open champion Li Na was snapped in the first round by Romanian teenager Simona Halep.

Ireland's Conor Niland threw up and gave up just 44 minutes into the first round match against the world No. 1 due to his food poisoning after eating a meal at a local restaurant.

"I really don't mind that I spend less time on the court," Djokovic said.

"Now, more than ever, I know that I can actually perform equally well on any surface," he added.

Fifth-seeded Li suffered the second successive opening round exit at the Flushing Meadow, after committing 54 unforced errors, 35 more than her 53th-ranked opponent Halep. The 19-year-old won the match in 6-3, 7-5.

"I was feeling tennis just too tough for me now," said Li. "I want to de well after Roland Garros, but always lose early. Now I even lose all the confidence on the court."

Li's elimination marked a second round of U.S. Open without a reigning Grand Slam singles champion remaining, just the first since 1971.

On Monday, the Wimbledon champion, Petra Kvitova from Czech Republic, also made an early exit.

Caroline Wozniacki, the women's world No. 1 demolished unseeded Spaniard Nuria Llagostera Vives 6-3 6-1.

No. 4 Victoria Azarenka wasted little time in dispatching Johanna Larsson of Sweden 6-1, 6-3.

In men's draw, fifth-seeded David Ferrer rallied from a set down to beat 78th-ranked Igor Andreev, who committed 53 unforced errors.

The Spaniard won 2-6, 6-3, 6-0, 6-4. He hasn't lost in the first round at a major tournament since 2005. Ferrer had 14 unforced errors in dropping the first set, then had only 24 the rest of the way.

11th seeded Jo-Wilfried Tsonga of France went through Lu Yen-Hsun of Chinese Taipei, 6-4, 6-4, 6-4.

19th-seed Spaniard Fernando Verdasco fought back from a set down, dismissing Jarkko Nieminen of Finland, 3-6, 6-4, 6-1, 6-4.

Other seeds progressed were 17th-seeded Jurgen Melzer of Austria, beating Eric Prodon of France, 6-2, 7-6(4), 6-2; 30th-seeded Ivan Ljubicic of Croatia, dispatching Blaz Kavcic of Slovenia, 7-6(5), 3-6, 6-1, 7-5

and 14th-seeded Swiss Stanislas Wawrinka, who rallied past Maximo Gonzalez of Argentina 3-6, 6-4, 6-1, 6-3.

Russia's Nikolay Davydenko powered off 32th-seeded Ivan Dodig of Croatia 6-7(6), 6-3, 6-0, 2-6, 6-2.

Another Russian Mikhail Youzhny, seeded 16th, become the highlight victim on Tuesday, losing to Ernests Gulbis of Latvia 6-2, 6-4, 6-4.

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