Australian Open champion Rybakina, of Kazakhstan, began her hardcourt run ahead of the US Open by outlasting Australian Daria Kasatkina 6-3, 5-7, 6-4 in 2hr 33min.

World number two Rybakina began the match by hitting six consecutive winners but also made 58 unforced errors.

"I want to see better serve, the percentage," Rybakina said after coughing up 16 double faults against 15 aces.

"Today in key moments it worked, but still the percentage is quite low.

"I'm still adapting a bit to the jet lag and everything, so maybe my energy level could be better."

Rybakina hit back-to-back double faults to drop the second set and three double faults while serving for the match before advancing to face 28th-seeded American Ann Li, who eliminated Canadian wildcard Kayla Cross 7-6 (7/2), 6-3.

"For the first match it wasn't too bad, but of course I want to improve," Rybakina said.

Pegula, who lost in a rain-delayed Washington final on Monday, was also extended in a 3-6, 6-3, 6-2 win over Poland's Magdalena Frech.

"I was just happy with the way I was able to mentally stay in there," said Pegula, who also beat Frech in the second-round in Washington a week ago.

"It was more of a mental battle just for me figuring out what to do and not get too frustrated.

"Having to face an opponent like that back-to-back is never easy, especially with a quick turnaround this week."

Pegula faces a third-round match against Uzbekistan's Kamilla Rakhimova, whose opponent Katerina Siniakova retired when trailing 6-7 (1/7), 6-0, 5-2.

Two-time Grand Slam winner Gauff cruised into the third round by dispatching fellow American Kayla Day 6-2, 7-5.

"The goal is to peak at the US Open, so I know maybe this first week I'm not going to be feeling 100 percent," said Gauff, who will face Greece's Maria Sakkari next.

"I feel like by tomorrow I'll be good. I think it was just a long couple of weeks for me training."

World number five Russian Mirra Andreeva, the French Open champion, cruised past Czech Karolina Pliskova 6-0, 6-2.

Andreeva will play Canada's 30th seed Leylah Fernandez, who beat Mexico's Renata Zarazua 6-2, 6-2.

Japan's Naomi Osaka, a four-time Grand Slam winner, won 6-2, 6-4 against Hungary's Panna Udvardy and will play Belgian Elise Mertens next.

Wimbledon champion Linda Noskova was knocked out by American Caty McNally 7-6 (7/5), 6-1.

McNally will meet Alexandra Eala, fresh from the Filipina's maiden WTA title in Washington. Eala beat Alycia Parks 6-1, 4-6, 6-2.