Serbia's Novak Djokovic celebrates winning his Quarter Final singles match against Canada's Denis Shapovalov after the match ATP Cup at Ken Rosewall Arena, Sydney, Australia, January 10, 2020. Serbia's Novak Djokovic battled through a stirring challenge from rising Canadian star Denis Shapovalov on Friday to propel his country into the semi-finals of the inaugural ATP Cup team event in Sydney with a 4-6 6-1 7-6(4) victory. REUTERS/Edgar Su

Serbia's Novak Djokovic in action during his Quarter Final singles match against Canada's Denis Shapovalov during the match ATP Cup at Ken Rosewall Arena, Sydney, Australia, January 10, 2020. Djokovic's fifth victory over the 20-year-old Canadian in as many clashes but by far the toughest, after Shapovalov controlled significant portions of the match played in Australia's intense summer heat. REUTERS/Ciro De Luca

Serbia's Novak Djokovic in action during his Quarter Final singles match against Canada's Denis Shapovalov during the match ATP Cup at Ken Rosewall Arena, Sydney, Australia, January 10, 2020. REUTERS/Edgar Su

Serbia's Novak Djokovic in action during his Quarter Final singles match against Canada's Denis Shapovalov during the match ATP Cup at Ken Rosewall Arena, Sydney, Australia, January 10, 2020. REUTERS/Ciro De Luca

Serbia's Novak Djokovic in action during his Quarter Final singles match against Canada's Denis Shapovalov during the match ATP Cup at Ken Rosewall Arena, Sydney, Australia, January 10, 2020. REUTERS/Edgar Su

Serbia's Novak Djokovic in action during his Quarter Final singles match against Canada's Denis Shapovalov during the match ATP Cup at Ken Rosewall Arena, Sydney, Australia, January 10, 2020. REUTERS/Edgar Su