Wimbledon champion Simona Halep's New York frustrations continued as the Romanian fourth seed suffered a shock 2-6 6-3 7-6(4) loss to American qualifier Taylor Townsend in second-round action on Thursday.

Halep, who fell in the first round here in the last two years, managed to fight off a pair of match points in the 10th game of the decider but Townsend's aggressive play at the net carried her home.

Townsend, 23, used an aggressive, net-rushing style of play to kept the pressure on Halep, who grew more and more frustrated as she was unable to come up with the necessary passing shots.

Halep arrived at Flushing Meadows looking to become the first woman since Serena Williams in 2012 to follow a Wimbledon win with a U.S. Open triumph.

Up next for Townsend in her first third round appearance at the U.S. Open, will be Romanian Sorana Cirstea, who beat Spain's Aliona Bolsova earlier on Thursday. 

Two-time Wimbledon winner Petra Kvitova was another surprise second-round casualty, the sixth seed losing 6-4 6-4 to Andrea Petkovic.

"I think that she likes my game, to be honest," said the Czech. "She played great. She moved pretty well as well.

"I had my chances, but I was always missing the last things. I was a break up in the second, and I should make it.

"But, yeah, overall I don't think I really played that bad. That's how it is. I was really trying hard. I was fighting." 

Meanwhile Naomi Osaka says she's over her first-night nerves as her bid to defend the title gathers pace.

Osaka needed three sets to dispatch Anna Blinkova in her first-round match, but there was no such drama in her 6-2 6-4 win over Magda Linette.

"For me, going into this match I was very calm, because I was thinking there is no way my nerves could be worse than they were the day before," she said.

"I also had that comfort of knowing I have played Magda before. I'd never played Blinkova before, so there is also that added stress not knowing what she can do.

"That stress was taken away today. I came in pretty confident with my abilities and I think that showed a little bit."