Serbia's Novak Djokovic kisses the trophy after he defeated South Africa's Kevin Anderson in the final match of the Mubadala World Tennis Championship in Abu Dhabi, United Arab Emirates, Saturday, Dec. 29, 2018. [Photo: AP]

Serbia's Novak Djokovic returns the ball to South Africa's Kevin Anderson during the final match of the Mubadala World Tennis Championship in Abu Dhabi, United Arab Emirates, Saturday, Dec. 29, 2018. [Photo: AP]

Serbia's Novak Djokovic reacts after he got a point against South Africa's Kevin Anderson during the final match of the Mubadala World Tennis Championship in Abu Dhabi, United Arab Emirates, Saturday, Dec. 29, 2018. [Photo: AP]