Japan's Naomi Osaka walks out onto court for her match against Britain's Katie Boulter during a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Andy Brownbill)

Russia's Daniil Medvedev hits the ball after defeating Japan's Kei Nishikori in their ATP Cup match in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Hamish Blair)

An Australian Open courtesy car waits outside the Grand Hyatt hotel in central Melbourne, Australia, Thursday, Feb. 4, 2021. All competition at six Australian Open tuneup events scheduled for Thursday was called off after a worker at one of the tournaments' Melbourne quarantine hotels tested positive for COVID-19. (AP Photo/Hamish Blair)

A pedestrian walks outside the Grand Hyatt hotel in central Melbourne, Australia, Thursday, Feb. 4, 2021. All competition at six Australian Open tuneup events scheduled for Thursday was called off after a worker at one of the tournaments' Melbourne quarantine hotels tested positive for COVID-19. (AP Photo/Hamish Blair)

United States' Serena Williams makes a forehand return to Bulgaria's Tsvetana Pironkova during a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Andy Brownbill)

Spectators walk around Melbourne Park in Melbourne, Australia, Wednesday, Feb. 3, 2021. The Australian Open site has the feel of a major this week with six tournaments being contested at Melbourne Park. Usually the tuneup tournaments Down Under are spread around the capital cities and some of the biggest stars have time off competitive play in the week before the first Grand Slam of the season. Not this year. The COVID-19 pandemic has changed the entire preparation for the Australian Open. (AP Photo/Hamish Blair)

Australia's Ash Barty makes a forehand return to Marie Bouzkova of the Czech Republic during a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Andy Brownbill)

United States' Sofia Kenin makes a forehand return to compatriot Jessica Pegula during tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Andy Brownbill)

Japan's Naomi Osaka makes a forehand return to Britain's Katie Boulter during a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Andy Brownbill)

Germany's Alexander Zverev makes a forehand return to Canada's Denis Shapovalov during their ATP Cup match in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Hamish Blair)

Spectators wave Greek and Australian flags as they watch Australia's Alex de Minaur and Greece's Stefanos Tsitsipas in their ATP Cup match in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Hamish Blair)