Day was already assured of regaining the world No. 1 ranking from American Jordan Spieth .
Day sank a 12 - foot par putt at the final hole to beat defending champion Rory McIlroy one - up in a morning semi-final .
Day , who sank a 12 - foot par putt at the final hole to beat defending champion Rory McIlroy one - up in a morning semi-final , was already assured of regaining the world No. 1 ranking from American Jordan Spieth .