Double Grand Slam winner Simona Halep was upended by a qualifier on Monday’s opening day of the US Open, with the Romanian losing to Ukrainian Daria Snigur 6-2, 0-6, 6-4.

The 20-year-old winner ranked 124 and playing her first match at a major and her third at the WTA level, refused to be intimidated by her opponent, finishing off the upset in less than one and three-quarter hours.

Halep continued a run of poor results at Flushing Meadows, going out in the first round for the fourth time in a dozen participations.

Daria Kasatina loses her first round match to Brit Harriet Dart. Photo:Roger Parker International Sports Fotos Ltd

“I’m very happy,” the surprised winner said. “I’m very, very, very nervous. But I tried to do my best.”

Halep has never had it easy in New York, with her career best a semi-final seven years ago.

She came to the final major of the season with wins in 19 of her last 22 matches, back into the Top 10 and bidding to win 40 matches in a single season for the first time since 2018 (now 39-11 for 2022).

Snigur, junior Wimbledon winner in 2019, is one of five Ukrainians in the draw

The youngster led 5-1 in the final set and suddenly was faced with a Halep fightback to 4-5.

But the outsider held her nerve, serving out the upset from a Halep error and shedding a few tears of joy.

Despite breaking the seventh seed’s serve five times from 15 chances, Snigur managed just 11 winners and 27 unforced errors.

Third seed Maria Sakkari fulfilled expectations as she beat Wimbledon semi-finalist Tatjana Maris of Germany 6-4, 3-6, 6-0 with six ace eight double-faults and five breaks of serve.

Maria Sakkari moves into the second round , after beating Tatjana Maris of Germany. Photo: Darren Carroll/USTA

“She’s very tricky and has a very different game than all the other girls on the Tour,” the Greek winner said.

“She’s very experienced and has been around, she’s a mom of two, so she’s more aware of things. That makes her a very tough opponent.”

American Coco Gauff, seeded 12th, got out of the blocks quickly at her home Grand Slam defeating Leolia Jeanjean of France 6-2, 6-3.

Victory marked the first time for the18-year-old on the showcase Ashe arena.

The winner ended with eight aces while never facing a break point.

“I was really nervous, I was feeling the pressure,” Gauff said. “It’s my first back in the home slam after a final.

“I didn’t think I was going to be on Ashe today. Opening the women’s draw on Ashe, too, at that.

“But it is nice to break the barrier. I really didn’t think about it until after the match was over and then I walked out [to salute] the crowd and said, Wait, I haven’t done this before. That’s why I told them it was my first win on Ashe.”

Gauff advanced in 80 minutes on the back of 34 winners.

Gauff also backed herself when coming forward. The current doubles No.1 came into the net 22 times and won all but five.

“Today was a good match for me,” Gauff said. “I think for the first round usually I’m nervous, but the way I played, I don’t think I felt that way.”

Wimbledon finalist Ons Jabeur of Tunisia reached the second round over American Madison Brengle 7-5, 6-2 while Brit Harriet Dart earned one of the best wins of her career with a defeat of Daria Kasatkina 7-6(8), 1-6, 6-3.

Harriet Dart wins her first round match. Phot: Roger Parker International Sports Fotos Ltd

The 88th-ranked Dart earned her first main draw win in New York as she put out the world No. 9.

Kasatkina came in with a pair of titles on North American hardcourt this season

Kasatkina, who won two titles in North America this summer. was upset in a match featuring 10 breaks of serve in two and a half hours on court.

“I’m a little bit in shock right now,” Darto told British media, Physically I don’t feel great but I really tried to stick at it and push through.

“I can’t believe I got the win.”

A pair of Canadians reached the second round, with Rebecca Marino beating Poland’s Magda Frech 6-2, 6-3 and former tournament winner Bianca Andreescu stopping France’s Harmony Tan 6-0, 3-6, 6-1.