Klitschko is the only heavyweight boxer to have reigned as world champion in three different decades. Along with Oliver McCall and Nikolai Valuev, he is also the only world heavyweight champion to have never been knocked down in any fight. Klitschko and George Foreman are the only heavyweight boxers in history to defend a world title after turning 40. Being the first professional boxing world champion to hold a PhD degree, and in reference to his punching power, Klitschko's nickname was "Dr. Ironfist".