Greece's Stefanos Tsitsipas changes his shirt during his men's singles match against Serbia's Viktor Troicki on day three of the Australian Open tennis tournament in Melbourne on January 16, 2019. (Photo by Greg Wood / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read GREG WOOD/AFP/Getty Images)
France's Benoît Paire avoids being hit by the ball from a shot by Austria's Dominic Thiem during their first round men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by WILLIAM WEST / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read WILLIAM WEST/AFP/Getty Images)
Serbia's Novak Djokovic reacts after a point against Mitchell Krueger of the US during their men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Paul Crock / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read PAUL CROCK/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: A general view inside Rod Laver Arena during the first round match between Novak Djokovic of Serbia and Mitchell Krueger of the United States on day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Julian Finney/Getty Images)
Slovakia's Martin Klizan waits for a line call after a point against France's Jo-Wilfried Tsonga during their men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Greg Wood / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read GREG WOOD/AFP/Getty Images)
TOPSHOT - France's Alize Cornet plays a forehand return to Spain's Lara Arruabarrena-Vecino during their first round women's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Greg Wood / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read GREG WOOD/AFP/Getty Images)
Russia's Daniil Medvedev receives medical treatment after suffering a nosebleed during his men's singles match against South Africa's Lloyd Harris on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Greg Wood / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read GREG WOOD/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: A general view of Melbourne Park during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Scott Barbour/Getty Images)
Romania's Simona Halep celebrates her victory against Estonia's Kaia Kanepi during their women's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by WILLIAM WEST / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read WILLIAM WEST/AFP/Getty Images)
Canada's Milos Raonic (L) shakes hands with Australia's Nick Kyrgios after winning their men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by DAVID GRAY / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read DAVID GRAY/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: The crowd reach for the wristbands of Novak Djokovic of Serbia after his first round match against Mitchell Krueger of the United Statesduring day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Mark Kolbe/Getty Images)
Serbia's Novak Djokovic celebrates his victory against Mitchell Krueger of the US during their men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Paul Crock / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read PAUL CROCK/AFP/Getty Images)
Serbia's Novak Djokovic hits a return against Mitchell Krueger of the US during their men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Paul Crock / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read PAUL CROCK/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: Novak Djokovic of Serbia reacts in his first round match against Mitchell Krueger of the United States during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Mark Kolbe/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: Laura Siegemund of Germany celebrates after winning match point in her first round match against Victoria Azarenka of Belarus during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Quinn Rooney/Getty Images)
A supporter of Japan's Misaki Doi wears a pair of sunglasses with the image of Winnie the Pooh as he cheers her on during her women's singles match against Madison Brengle of the US on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Greg Wood / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read GREG WOOD/AFP/Getty Images)
Croatia's Ivo Karlovic celebrates his victory against Poland's Hubert Hurkacz during their men's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Greg Wood / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read GREG WOOD/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: Serena Williams of the United States looks on in her first round match against Tatjana Maria of Germany during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Mark Kolbe/Getty Images)
Serena Williams of the US (L) shakes hands with Germany's Tatjana Maria after winning their women's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by Paul Crock / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read PAUL CROCK/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: Serena Williams of the United States shakes hands with Tatjana Maria of Germany following her victory in the first round during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Fred Lee/Getty Images)
Serena Williams of the US reacts after a point against Germany's Tatjana Maria during their women's singles match on day two of the Australian Open tennis tournament in Melbourne on January 15, 2019. (Photo by WILLIAM WEST / AFP) / -- IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE -- (Photo credit should read WILLIAM WEST/AFP/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: Portuguese chair umpire, Carlos Ramos looks on Rod Laver Arena during the first round match between Novak Djokovic of Serbia and Mitchell Krueger of the United States during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Mark Kolbe/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: A young fan cool off during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Mike Owen/Getty Images)
MELBOURNE, AUSTRALIA - JANUARY 15: Serena Williams of the United States reacts in her first round match against Tatjana Maria of Germany during day two of the 2019 Australian Open at Melbourne Park on January 15, 2019 in Melbourne, Australia. (Photo by Fred Lee/Getty Images)(Photo by Fred Lee/Getty Images)