Russia's ATP Cup winners Daniil Medvedev,right, Andrey Rublev second left, Aslan Karatsev and Evgeny Donsky, left, wave to the crowd after defeating Italy in the final in Melbourne, Australia, Sunday, Feb. 7, 2021.(AP Photo/Hamish Blair)

Serbia's Novak Djokovic makes a forehand return to Germany's Alexander Zverev during their ATP Cup match in Melbourne, Australia, Friday, Feb. 5, 2021.(AP Photo/Hamish Blair)

Serbia's Novak Djokovic signs autographs after defeating Germany's Alexander Zverev in their ATP Cup match in Melbourne, Australia, Friday, Feb. 5, 2021.(AP Photo/Hamish Blair)

Serbia's Novak Djokovic celebrates after defeating Germany's Alexander Zverev in their ATP Cup match in Melbourne, Australia, Friday, Feb. 5, 2021.(AP Photo/Hamish Blair)

United States' Serena Williams, right, is congratulated by compatriot Danielle Collins following their match at a tuneup event ahead of the Australian Open tennis championships in Melbourne, Australia, Friday, Feb. 5, 2021.(AP Photo/Andy Brownbill)

Spain's Garbine Muguruza rests during her match against Australia's Ash Barty in the final of the Yarra River Classic in Melbourne, Australia, Sunday, Feb. 7, 2021.(AP Photo/Andy Brownbill)

Australia's Ash Barty, left, is congratulated by Spain's Garbine Muguruza after winning the final of the Yarra River Classic in Melbourne, Australia, Sunday, Feb. 7, 2021.(AP Photo/Andy Brownbill)

Greek fans cheer on Greece's Stefanos Tsitsipas during his ATP Cup match against Australia's Alex de Minaur in Melbourne, Australia, Wednesday, Feb. 3, 2021. (AP Photo/Hamish Blair)

Spain's Rafael Nadal signs autographs for fans on Rod Laver Arena during the ATP Cup in Melbourne, Australia, Tuesday, Feb. 2, 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)