Serbia players hold up the ATP CUP after defeating Spain during their ATP Cup tennis tournament in Sydney.

Novak Djokovic of Serbia falls to the ground after defeating Daniil Medvedev of Russia during their ATP Cup semifinal tennis match in Sydney.

Novak Djokovic of Serbia falls to the ground after defeating Daniil Medvedev of Russia during their ATP Cup semifinal tennis match in Sydney

Rafael Nadal, front, and Pablo Carreno Busta of Spain celebrate winning a point against Sander Gille and Joran Vliegen of Belgium during their ATP Cup tennis match in Sydney.

Spain's Rafael Nadal practices in Sydney ahead of Friday's ATP Cup tennis match in Sydney.

Novak Djokovic of Serbia waves after he won his match against Cristian Garin of Chile at the ATP Cup tennis tournament in Brisbane, Australia.

Novak Djokovic of Serbia reacts after winning the first set during his match against Kevin Anderson of South Africa at the ATP Cup tennis tournament in Brisbane, Australia.

Novak Djokovic of Serbia plays a shot during his match against Kevin Anderson of South Africa at the ATP Cup tennis tournament in Brisbane, Australia

Guido Pella of Argentina reacts after beating Kamil Majchrzak of Poland during their ATP Cup tennis match in Sydney

Denis Shapovalov of Canada reacts after he won his match against Stefanos Tsitsipas of Greece at the ATP Cup tennis tournament in Brisbane, Australia.