British No 1 Katie Boulter is out of the National Bank Open in Toronto after defeat to world No 3 Aryna Sabalenka, while top seed Coco Gauff suffered a shock loss to Diana Shnaider.

Boulter was unable to take any of the eight break points she manufactured in a competitive match and Sabalenka converted three from three to win 6-3 6-3 and advance to the quarter-finals.

Sabalenka will next face Amanda Anisimova, one of five Americans in the last eight alongside defending champion Jessica Pegula, Peyton Stearns, Emma Navarro and Taylor Townsend.

Reigning US Open champion Gauff missed out on making that six, though, after going down 6-4 6-1 to Shnaider as her Russian opponent picked up a first career win over a top-10 player.

World No 24 Shnaider – who broke Gauff at 5-4 in the opening set and then twice in the second – will play last year’s runner-up Liudmila Samsonova in the quarter-finals in Canada.

Gauff will return to action at the Cincinnati Open next week as she tries to win that tournament for the second year running, while she will then aim to defend her US Open title from August 26.

The world No 2 said of her defeat to Shnaider: “I just wasn’t finding my timing well. I had a lot of unforced errors, a lot of double faults. She played steady and she didn’t give me any free points.

“I question whether I should have played (in Toronto) or not, but I wanted to test myself.

“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 said: “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.

“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!'”

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• Winston-Salem Open (ATP 250) – 18-24 August
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