Serena Williams of the United States celebrates after the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Serena Williams of the United States returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Serena Williams (R) of the United States shakes hands with Marie Bouzkova of the Czech Republic after their women's singles semifinal match at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Marie Bouzkova of the Czech Republic returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Serena Williams of the United States returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Serena Williams of the United States returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Marie Bouzkova of the Czech Republic returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Serena Williams of the United States returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Marie Bouzkova of the Czech Republic returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Marie Bouzkova of the Czech Republic returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Marie Bouzkova of the Czech Republic returns the ball during the women's singles semifinal match between Marie Bouzkova of the Czech Republic and Serena Williams of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Bianca Andreescu of Canada celebrates during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Sofia Kenin of the United States returns the ball during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Bianca Andreescu of Canada celebrates after the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Bianca Andreescu of Canada returns the ball during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Bianca Andreescu of Canada returns the ball during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Bianca Andreescu of Canada returns the ball during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Sofia Kenin of the United States returns the ball during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)

Bianca Andreescu of Canada returns the ball during the women's singles semifinal match between Bianca Andreescu of Canada and Sofia Kenin of the United States at the 2019 Rogers Cup in Toronto, Canada, Aug. 10, 2019. (Xinhua/Zou Zheng)