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SYDNEY, AUSTRALIA - JANUARY 12: Novak Djokovic plays a volley while team mate Viktor Troicki of Serbia watches on during their final doubles match against Feliciano Lopez and Pablo Carreno Busta of Spain on day 10 of the ATP Cup at Ken Rosewall Arena on January 12, 2020 in Sydney, Australia. (Photo by Cameron Spencer/Getty Images)

SYDNEY, AUSTRALIA - JANUARY 12: Novak Djokovic plays a volley while team mate Viktor Troicki of Serbia watches on during their final doubles match against Feliciano Lopez and Pablo Carreno Busta of Spain on day 10 of the ATP Cup at Ken Rosewall Arena on January 12, 2020 in Sydney, Australia. (Photo by Cameron Spencer/Getty Images)