Spain's Garbine Muguruza makes a forehand return to Marketa Vondrousova of the Czech Republic during a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Saturday, Feb. 6, 2021.(AP Photo/Andy Brownbill)

Germany's Alexander Zverev, left, talks with a trainer during his match against Russia' Daniil Medvedev in their ATP Cup semifinal match in Melbourne, Australia, Saturday, Feb. 6, 2021.(AP Photo/Hamish Blair)

Spain's Garbine Muguruza walks from the court after defeating Marketa Vondrousova of the Czech Republic in a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Saturday, Feb. 6, 2021.(AP Photo/Andy Brownbill)

Japan's Naomi Osaka is interviewed after defeating Romania's Irina-Camelia Begu at a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Friday, Feb. 5, 2021.(AP Photo/Andy Brownbill)

Russia' Daniil Medvedev makes a forehand return to Germany's Alexander Zverev in their ATP Cup semifinal match in Melbourne, Australia, Saturday, Feb. 6, 2021.(AP Photo/Hamish Blair)

Germany's Alexander Zverev holds his back during his match against Russia's Daniil Medvedev in their ATP Cup semifinal match in Melbourne, Australia, Saturday, Feb. 6, 2021.(AP Photo/Hamish Blair)

United States' Serena Williams reacts during her match against compatriot Danielle Collins during a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Friday, Feb. 5, 2021.(AP Photo/Andy Brownbill)