Nadal, the defending U.S. Open champion and this year's second-seeded player, didn't turn in a starring performance, but he managed through Kazakhstan's Andrey Golubev 6-3, 7-6 (1) and 7-5 in almost three hours.

　　Djokovic enjoyed a comfortable walk through into the second round as his opponent Conor Niland retired trailing 6-0, 5-1 for food poisoning, who threw up and pulled out of the tournament in just 44 minutes.

　　American favorite contender Serena Williams, playing her first match in New York for two years because of injuries, powered through the night session match 6-1, 6-1 over Serbian Bojana Jovanovski in 56 minutes.

　　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 dewell after Roland Garros, but always lose early. Now I even lose all the confidence on the court."

　　Golubev, who was ranked 98thin the world and who had gone through a stretch of losing 17 straight matches this season, was leading Nadal, 4-2, in the second set after breaking the Spaniard at love and was making Nadal cranky by pounding winners from both sides and bringing Nadal forward with tricky drop shots.

　　"I was a little lucky to win in straight sets," Nadal said. "He was a very difficult player to play. He played very fast."

　　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.