A native of Gdansk, Poland, Michalczewski came up through Poland's state-run sports program as a boy and had a successful amateur career before turning professional in 1991. Fighting mostly in neighboring Germany, he began piling up wins and was soon signed by Universum Boxing Promotions, one of the leading boxing promoters in Europe. His aggressive style earned him the nickname The Tiger.
He won the German national light heavyweight (175-pound) title early in 1993 and added an IBF regional belt later that year. Finally, on September 10, 1994, Michalczewski captured the WBO world light heavyweight title with a 12-round decision over Leeonzer Barber.
Between then and March of 2003, Michalczewski made 23 successful defenses of his WBO title and picked up three other belts along the way. Three months after beating Barber, he won the WBO's cruiserweight (190-pound) title with a knockout of Nestor Giovannini. However, he soon gave up that title to continue campaigning as a light heavyweight.
On June 13, 1997, he won recognition by many as the world's top light heavyweight when he outpointed Virgil Hill over 12 rounds to add Hill's WBA and IBF titles to his own. However, Michalczewski soon lost both titles. The WBA stripped him for displaying its belt along with that of the WBO, an organization it doesn't recognize. The IBF did so a few weeks later, when Michalczewski refused to defend his title against a mandatory challenger within 30 days.
After beating Hill, Michalczewski knocked out 14 consecutive opponents, all in defense of his WBO title. However, he drew some criticism for never fighting outside Germany, and also for never facing Roy Jones Jr. Shortly after Michalczewski beat Hill, Jones moved up to light heavyweight and began to methodically capture each of the other three major belts in the division. Boxing fans began to clamor for the two men to meet one another in the ring, but it never happened. Universum was reluctant to make a fight for Michalczewski, a huge draw in Germany, anywhere else. Meanwhile, Jones still harbors bad memories of being robbed of a gold medal in the 1988 Olympic Games and prefers to fight only in his native United States.
Even without facing Jones, Michalczewski has posted some impressive victories, including those over Hill and former world champions Graciano Rocchigiani and Montell Griffin.
Michalczewski had a perfect record of of 48 wins and no losses when he faced Julio Cesar Gonzalez of Mexico in defense of his title on October 18, 2003. Coming into the fight, he was within one victory of tying Rocky Marciano's all-time record for an undefeated record by a world championship boxer. A win also would have put him just one victory short of Joe Louis' all-time record for successful defenses at any weight class. However, Michalczewski was unable to pull it off. He lost a split decision to Gonzalez at the Color Line Arena in Hamburg, Germany and his career record dropped to 48-1.
Despite the loss, he still holds the record for the most consecutive successful title defenses at light heavyweight.