Novak Djokovic of Serbia (left) and Fernando Verdasco of Spain (right) listen to Djokovic’s coach Boris Becker during a joint practice session ahead of the Australian Open tennis tournament at Melbourne Park in Melbourne, Australia, 15 January 2016. EPA/JULIAN SMITH

Retiring Australian player Lleyton Hewitt is seen during a joint practice session with Swiss player Roger Federer on Rod Laver Arena ahead of the Australian Open Tennis tournament in Melbourne, Australia, 15 January 2016. EPA/JULIAN SMITH

Swiss player Roger Federer (right) and retiring Australian player Lleyton Hewitt take a break during a joint practice session ahead of the Australian Open Tennis tournament in Melbourne, Australia, 15 January 2016. EPA/JULIAN SMITH

Australian Open reigning champions Novak Djokovic of Serbia (left) and Serena Williams (left) of the United States pose for a photograph with their trophies at the official draw of the 2016 Australian Open Tennis tournament in Melbourne, Australia, 15 January 2016. EPA/JULIAN SMITH

Japanese player Kei Nishikori during a practice session ahead of the Australian Open Tennis tournament in Melbourne, Australia, 15 January 2016. EPA/JULIAN SMITH