TORONTO — Top-seeded Coco Gauff tumbled out of the National Bank Open on Friday, falling 6-4, 6-1 to 14th-seeded Diana Shnaider in windy conditions at Sobeys Stadium.

Preparing for her U.S. Open title defense, Gauff was broken trailing 5-4 in the first set. The 20-year-old American double-faulted down 2-1 in the second and Shnaider held serve to go up 4-1.

“I just wasn’t finding my timing well,” said Gauff, the No. 2-ranked player in the world. “I had a lot of unforced errors and a lot of double faults. She played steady and she didn’t give me any free points.”

She was playing her first hard-court event since April.

“I question whether I should have played (in Toronto) or not, but I wanted to test myself,” Gauf said. “I didn’t have high expectations, but I wish I could have competed better today, even if it resulted in a loss. I don’t think I competed well.”

Shnaider, also 20, sealed the victory with another break to set up a quarterfinal match against No. 6 Liudmila Samsonova, a 6-2, 6-4 winner over Elise Mertens.

“I know Coco is very physically prepared for everyone on tour, and she’s running very well, she’s covering all of the balls and all the angles on the court,” Shnaider said. “So when there were long rallies and I was winning them I was like, ‘Wow, I just beat Coco in long rallies. I am good today.’ I tried to play it cool, but inside I was like, ‘Yes! I made it!'”

In the late match, No. 2 Aryna Sabalenka beat Katie Boulter 6-3, 6-3.

Earlier, defending champion Jessica Pegula beat qualifier Ashlyn Krueger 6-2, 6-4.

The third-seeded Pegula, from nearby Buffalo, New York, took advantage of Krueger’s nine double faults. Last year in Montreal, Pegula beat Samsonova in the final.

She will face Peyton Stearns, who became the fifth American in the quarterfinals when No. 12 Victoria Azarenka retired because of a thigh injury.

Taylor Townsend topped fourth-seeded Jelena Ostapenko 6-2, 6-1 and No. 8 Emma Navarro beat No. 11 Marta Kostyuk 7-5, 7-5 to set up a quarterfinal pairing.

Amanda Anisimova advanced after No. 10 Anna Kalinskaya of Russia retired because of dizziness. Anisimova took the first set 6-2. She will face Sabalenka in the quarterfinals.