NEW YORK, NY, USA - Lesia Tsurenko surged to an emphatic upset over World No.2 Caroline Wozniacki, stunning the Australian Open champion, 6-4, 6-2 to end the night session on Louis Armstrong Stadium.

A two-time champion at the Abierto Mexicano Telcel, Tsurenko has been on site since the first day of the Qualifying Tournament to support fellow Ukrainian Marta Kostyuk, and has been playing some of her best tennis this summer. 

She knocked out two-time Grand Slam champion Garbiñe Muguruza at the Western & Southern Open for her first Top 10 win since 2015, and earned another against Wozniacki on Thursday night, securing victory after one hour and 41 minutes on court.

"I have a good experience of playing night matches with the high humidity because of two titles in Acapulco."

- Lesia Tsurenko

"I actually think that my win against Muguruza in Cincinnati and some victories there gave me the confidence that I can play well," Tsurenko said. "I was here early. I prepared well. I got many sessions with my coach and also with the other players playing points.

"I think I was completely ready for this tournament. Plus I took that confidence from the Cincinnati tournament. I finally feel good with my lower back. I had issues in Montréal and Cincinnati. So I feel really good."

Wozniacki has struggled with multiple injuries this summer, retiring from her match in Cincinnati with a left knee injury, but appeared in solid form on Tuesday when she beat 2011 champion Samantha Stosur in straight sets, and took a 3-1 lead to start her match against the Ukrainian veteran.

"I think I started off pretty well, then I think I played well in parts of the first set, just trying to stay aggressive," Wozniacki said after the match. "She was playing smarter than me. She played the game that I was supposed to be playing. She got a lot of balls back. She played with the angles. She waited for the short ball. When the short ball came, she played aggressive. She went back and kept the ball in play when she had to.

"I tried to play aggressive because I saw openings. I saw balls that I felt like I could finish off, I felt like I could hit big on.

"The court is a little bit slower. I should have made better adjustments. I just didn't. Then I was trying to play a little bit higher, maybe change the rhythm, but it just wasn't really going my way. I think the problem also was my serve and returns weren't great today. I feel like if maybe I'd served better, I could have put more pressure on her."

Tsurenko turned the tables from there, winning four straight games - including a marathon eighth game to consolidate her break of serve - and gamely served out the opening set.

"Today was a funny day because I had a nice warm-up in the morning, but then I was stuck in traffic. I was late for my warm-up. I was just hitting on Armstrong for 15 minutes, then they started the men's match. So I went to serve on the Practice court. At least I tried a little bit.

"Of course, I like the court so much, it's not so big as Ashe Stadium, but still very noisy. It was so humid today, like my towels are completely wet. It was really tough to hold the racquet.

"But I think the weather will be better. Maybe rain tomorrow, but not so hot as first round match. It was really hot on first match."

The former World No.29 only got stronger from there, nabbing the only two service breaks of the second set to clinch the upset and reach the third round of the US Open for the second time in two years.

In all, Tsurenko struck a solid 20 winners to 38 unforced errord - seven of which were double faults - while Wozniacki managed just six winners in two sets, ultimately undone by 33 unforced errors off the ground.

Up next for the 29-year-old is Czech youngster Katerina Siniakova. The top seed in doubles with compatriot Barbora Krejcikova, Siniakova survived a titanic three-setter against Ajla Tomljanovic, 6-4, 6-7(3), 7-6(4).

"I actually practiced with her right before the beginning of the tournament. I've never played match against her, I think. She's playing really flat. She's trying to be really aggressive on court. It will be a different game, for sure, than we had today. I'm sure that I can play in many ways. I will think more about it in two days."