(0:41)Open Extended ReactionsLONDON -- Oleksandr Usyk ruthlessly knocked out Daniel Dubois in Round 5 to become the undisputed world heavyweight champion again Saturday. Usyk won Dubois' IBF title, to add to his WBC, WBA and WBO belts, in a fifth title defence. Usyk first sent Dubois to the canvas with a right to the head for a count before flattening Dubois with a left hook. In Round 5, Usyk landed a left-right combination as Dubois began to look more open and a glancing right to the head sent Dubois to his knees for the first count. After Dubois missed with a right hand, Usyk uncorked a sweeping left hook to return the Englishman to the canvas.