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.

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