Romanian seventh seed Simona Halep suffered a shock first-round defeat at the US Open as she lost to Ukrainian qualifier Daria Snigur in three sets.

Having led the third set 5-1, the 20-year-old held her nerve to beat two-time major winner Halep 6-2 0-6 6-4.

It is a first Tour-level match win for the world number 124, in her first meeting with a top-10 ranked player.

Snigur could barely believe her achievement as Halep sent a backhand into the net to confirm a huge opening day upset at Flushing Meadows, New York.

Halep, who reached the Wimbledon semi-finals in July, lost just six points as she claimed a one-sided second set to level the match.

But, unfazed, a double break of serve in the third put Snigur on the verge of victory before she withstood a late fightback from her opponent and converted her third match point.

It was not long before tears of joy followed as she came back from 0-30 behind in the final game, while her father watched on in disbelief, resting his hands on his head, as his daughter spoke to the crowd.

"I'm very happy. It's impossible for me," said Snigur. "I have to say thanks to Simona for this incredible match. I want to thank all the fans who watched our match today.

"Of course this match is for Ukraine, for my family, for everyone who supported me. I want to say thank you to you all."

She will face Poland's Magdalena Frech or Canadian Rebecca Marino in round two.