Rafael Nadal from Spain, right, stands next to Stefanos Tsitsipas of Greece, celebrates after Nadal won the final match of the Mubadala World Tennis Championship in Abu Dhabi, Unit...

Stefanos Tsitsipas of Greece returns the ball to Rafael Nadal from Spain during the final match of the Mubadala World Tennis Championship in Abu Dhabi, United Arab Emirates

Rafael Nadal from Spain returns the ball to Stefanos Tsitsipas of Greece during the final match of the Mubadala World Tennis Championship in Abu Dhabi, United Arab Emirates

Novak Djokovic of Serbia returns the ball to Karen Khachanov from Russia during a match for the third place of the Mubadala World Tennis Championship in Abu Dhabi, United Arab Emir...