
Serbia's Novak Djokovic returns the ball to Spain's Albert Ramos-Vinolas during their Davis Cup quarterfinal tennis match in Belgrade on April 7, 2017.(Xinhua/Predrag Milosavljevic)

Serbia's Novak Djokovic returns the ball to Spain's Albert Ramos-Vinolas during their Davis Cup quarterfinal tennis match in Belgrade on April 7, 2017.(Xinhua/Predrag Milosavljevic)