Aryna Sabalenka threw a ball away in frustration.

The No. 3 player in women's tennis then slammed her racket to the court as emotions boiled over.

Sabalenka was down a set to an opponent she had struggled against in the past.

Things only got worse.

The Belarusian star was stunned 6-4, 6-2 by Amanda Anisimova of the U.S. in Saturday's National Bank Open quarterfinals at a windswept Sobeys Stadium.

Sabalenka, the two-time defending Australian Open champion, lost to the WTA Tour's 132nd ranked player for a fifth time in seven meetings.

Anisimova broke the tournament's No. 2 seed up 5-4 in the first set when Sabalenka fired long and took out some anger on her racket.

The 22-year-old American broke Sabalenka again for a 4-2 lead in the second set when she sent a shot into the net before Anisimova served out to take a 5-2 lead.

Sabalenka, who missed Wimbledon with a shoulder injury and skipped the Paris Olympics, committed her eighth and final double fault of the match down love-40 to seal Anisimova's first trip to a WTA 1000 semifinal.

She will face Emma Navarro on Sunday after the No. 8 seed downed U.S. counterpart Taylor Townsend 6-3, 7-6 (5).

"It was really tough," Navarro, who also made her first WTA 1000 semifinal, said of the conditions on a blustery, overcast afternoon. "The wind was swirling."

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Defending champion and No. 3 seed Jessica Pegula faced Peyton Stearns later Saturday in another all-American quarterfinal matchup.

The five U.S.-born women in the final eight are the most at a WTA 1000 event since the format's inception in 2009, and the most at any tour event since 2001.

"I had the opportunity to play Billie Jean King Cup earlier this year, and then the Olympics a couple weeks ago, and spend some more time with my fellow Americans," said Navarro, 23. "It's been really cool getting to know them better, and for sure seeing their success."

No. 14 Diana Shnaider, who knocked off top seed Coco Gauff of the U.S. on Friday, was scheduled to take on fellow Russian and No. 6 seed Liudmila Samsonova in the late match of the US$3.2-million event.

Pegula beat Samsonova in last year's final in Montreal.

Townsend's unlikely run as a "lucky loser" — the first to make a WTA 1000 quarterfinal — at Canada's tennis championship came to an end on Centre Court after entering the draw as an injury replacement.

She advanced when No. 16 seed Dayana Yastremska retired from their match before defeating wild-card Marina Stakusic of Mississauga, Ont., and stunning No. 4 Jelena Ostapenko.

"This week has been amazing," said the 28-year-old Townsend. "It's not anything shy of what I believe that I could do. I know that it's in my capability to capitalize on the opportunities that I have and play the tennis that I played, and even better."

All of Canada's singles entries were eliminated earlier in the week — both here in Toronto and at the men's event in Montreal — but the women's doubles bracket still had plenty of red Maple Leafs dotting the page entering Saturday.

Leylah Fernandez of Laval, Que., and younger sister Bianca beat France's Kristina Mladenovic and China's Zhang Shuai 5-7, 7-5 [12-10] in the quarters to guarantee a least one Canadian in Monday's final.

"We fought through from the first point all the way to the last," said Leylah Fernandez, who was seething after being eliminated from singles on Thursday. "I'm just happy that I was able to play doubles with my sister.

"We're in the semifinals together."

The siblings will face the top-seeded duo of Ottawa's Gabriela Dabrowski and New Zealand's Erin Routliffe after they came back to beat wild-card entry Ariana Arseneault of Richmond Hill, Ont., and Edmonton's Mia Kupres 3-6, 6-3 [10-4].

The finals at this year's NBO tournament are being played Monday instead of the traditional Sunday slot because of Olympic-related scheduling.

Navarro beats Townsend to make National Bank Open semifinals

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