Wozniacki, who only returned to tennis in August after retiring following the Australian Open in 2020 to start a family, booked her place in the last 32 with a 7-5, 7-6 (7/5) victory over the Czech Republic's Kvitova.

"I mean, being out here in Arthur Ashe Stadium night session playing in front of a packed crowd, I mean, this couldn't be better," an elated Wozniacki said after her win. 

"It's a dream come true. If you'd asked me three years ago I'd have said, 'I'll never be back here playing on this court.' But to be back and to beat to the world number 11 feels very, very special. 

"It really means so much to me. I love New York. I love this court. I love everything about the city and playing here. 

"As soon as I came here over a week ago, I already instantly felt so comfortable in these courts and knew that I could play some great tennis and I'd be dangerous in the draw."

The 33-year-old from Denmark, a US Open finalist in 2009 and 2014, will face world number 433 Jennifer Brady in the third round on Friday.

Wozniacki, whose last appearance at the US Open came in 2019, was handed a wild card into this year's tournament.

After battling to take a hard-fought first set, Wozniacki held two match points on Kvitova's serve at 5-4 in the second set.

Two-time Wimbledon champion Kvitova dug deep to hold and the set went with serve before Wozniacki sealed the tie-break 7/5.

Zhang makes history to send Ruud crashing out of US Open

Zhang Zhizhen sent fifth seed Casper Ruud crashing out of the US Open on Wednesday, creating tennis history by becoming the first man from China to beat a top-five player.

The 26-year-old from Shanghai -- ranked 67th in the world -- produced the match of his life to defeat last year's US Open finalist 6-4, 5-7, 6-2, 0-6, 6-2 in 3hr 19min.

Zhang's victory was the first time any male Chinese player has beaten a player ranked in the world's top five.

Zhang will face Australia's Rinky Hijikata in the third round on Friday, bidding to reach the last 16 of a Grand Slam for the first time.

Tempers flared after Ruud had levelled the match at two sets apiece with a 6-0 fourth set.

Zhang left the court for several minutes to take a toilet break and change clothes in a move that clearly disrupted Ruud's momentum.

The Norwegian was broken immediately in the first game of the final set, and a frustrated Ruud complained angrily at the chair umpire, accusing him of failing to enforce time limits.

"Why don't you do anything?" Ruud yelled at the umpire. "You follow the rules clearly some times then other times you don't give a shit. Why don't you do anything?".

With Ruud unravelling, Zhang rammed home his advantage. He broke again in the fifth game to take a 4-1 lead and then held for a 5-1 lead.

Ruud held serve to close the gap to 5-2, but Zhang would not be denied, producing a superb backhand volley on match point to seal victory.