sentences:  Having never been knocked down, Vitali Klitschko is widely regarded as the tougher fighter of the two.In fact, Vitali is the only heavyweight world champion to have never been knocked down in any fight, and alongside George Foreman he is the only heavyweight boxer in history to defend a world title after turning 40.Furthermore, his 87% knockout percentage is one of the best knockout-to-fight ratios of any champion in heavyweight boxing history.