Second seed Wozniacki, 28, broke the 2011 champion's serve five times on a day when temperatures in New York topped 32C with high humidity.

The Dane plays Lesia Tsurenko next.

Organisers implemented the extreme heat policy at Flushing Meadows with a 10-minute break between the second and third sets in women's singles matches.

There was a 10-minute break between the third and fourth sets of men's singles matches.

Wimbledon champion and fourth seed Angelique Kerber saw off Russian Margarita Gasparyan 7-6 (7-5) 6-3 and faces Swede Johanna Larsson in the second round.

Two-time Wimbledon champion Petra Kvitova is also into the second round after beating Belgian Yanina Wickmayer 6-1 6-4.

The Czech fifth seed, 28, broke serve twice to take the first set and won five successive games from 4-1 down in the second. She will play China's Yafan Wang next.

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Two-time US Open runner-up Wozniacki said: "I'm just glad to have won this one. It's definitely difficult. It's very hot out here. I just tried to cool down between games."

American 24th seed Coco Vandeweghe, a semi-finalist at Flushing Meadows last year, was knocked out in the first round by Belgian Kirsten Flipkens, 6-3 7-6 (7-3).

Latvian 10th seed Jelena Ostapenko, the 2017 French Open champion, beat Germany's Andrea Petkovic 6-4 4-6 7-5 to set up a second-round meeting with American Taylor Townsend.

Russian 11th seed Daria Kasatkina, who was a quarter-finalist at Wimbledon and the French Open, beat Hungary's Timea Babos 6-4 5-7 6-4 and faces Belarusian Aliaksandra Sasnovich in the second round.

Wimbledon quarter-finalist Kiki Bertens won 6-0 7-5 against Czech Kristyna Pliskova and will play American Francesca Di Lorenzo in round two.

Japan's Naomi Osaka, seeded 20th, defeated German Laura Siegemund 6-3 6-2 and will play Israel's Julia Glushko.

Spain's 30th seed Carla Suarez Navarro came from behind to beat American Nicole Gibbs 5-7 6-3 6-4 and will face France's Kristina Mladenovic in the next round.

Fourteenth seed Madison Keys will play fellow American Bernarda Pera after seeing off France's Pauline Parmentier 6-4 6-4.