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.
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.
In the late match, No. 2 Aryna Sabalenka played Katie Boulter.
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.
