Fourth-seeded Naomi Osaka of Japan made the semifinals of the Western & Southern Open on Wednesday but withdrew a few hours later in protest at racial injustice.
Her decision comes after protests erupted in the United States in light of Sunday's police shooting of Jacob Blake, a Black man from Kenosha, Wisconsin. Officers shot Blake seven times in the back in front of his three children.
In Wednesday's quarterfinals, Osaka beat Anett Kontaveit of Estonia 4-6, 6-2, 7-5 to reach the semifinals, where the two-time Grand Slam champion was due to face Elise Mertens of Belgium.
Victoria Azarenka of Belarus will play eighth-seeded Johanna Konta of Britain in the other women's singles semifinal. Konta beat Maria Sakkari of Greece 6-4, 6-3 and Azarenka advanced with a 7-6(9), 6-2 victory against Ons Jabeur of Tunisia.
In men's singles, world number one Novak Djokovic of Serbia defeated Jan-Lennard Struff of Germany 6-3, 6-1 to reach the last four. He will face Spain's Roberto Bautista Agut, who beat defending champion Daniil Medvedev from Russia 1-6, 6-4, 6-3.
In the other men's singles semifinal, Canadian star Milos Raonic will take on Greek Stefanos Tsitsipas.
In women's doubles, Xu Yifan of China paired up with American Nicole Melichar to beat the Russian pairing of Anna Blinkova and Veronika Kudermetova 6-7(6), 6-3, 10-7 to reach the semifinals.