If they are stepping in toward you, you have several options instead of ducking down. First, you can fire a jab as you step back, trying to keep seperation and distance inbetween you and the opponent. You could also slip, and step to the side causing your opponent to have to reset, and turn again to face you. Fighting while going backwards is something you may have to work on, as well as your footwork to help you maneuver around these situations.