In 1988, while competing in Germany for the Polish national team, Michalczewski defected from the team to stay in Germany. Becoming a citizen of Germany in 1991 and turning professional the same year, he was soon signed by Universum Box-Promotion, one of the leading boxing promoters in Europe. His aggressive style earned him the nickname "The Tiger."
In 1997, he defeated Virgil Hill over 12 rounds to add Hill's WBA and IBF titles to his own. However, Michalczewski soon lost both alphabet titles. The WBA stripped him for displaying its belt along with that of the WBO, an organization it didn't recognize. The IBF did so a few weeks later, when Michalczewski was unable to defend his title against mandatory challenger William Guthrie within 30 days.
After beating Hill, Michalczewski knocked out 14 consecutive opponents, all in defense of his WBO titles. On 1998, he defeated Drake Thadzi and in 2000 he went to defeat Graciano Rocchigiani.
He defended his WBO light heavyweight title 23 times and was a two weight division champion, winning titles as both a Light Heavyweight, and as a Cruiserweight.
Michalczewski had a perfect record of 48 wins and no losses when he faced Julio César González of Mexico in defense of his title on 18 October 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, the now 35-year-old Michalczewski was unable to pull it off. He lost a controversial split decision to the 27-year-old Gonzalez at the Color Line Arena in Hamburg, and his career record dropped to 48-1.
For years a bout with Roy Jones Jr., was discussed, but never happened due to various reasons. Both parties pointed the fingers towards each other for this. Michalczewski did defeat several notable fighters such as Montell Griffin, Graciano Rocchigiani, and Drake Thadzi. He lost his last two fights to Julio Cesar Gonzalez and Fabrice Tiozzo.
Michalczewski did defeat several notable fighters such as Montell Griffin, Graciano Rocchigiani, and Drake Thadzi.
133-15-2 (83 KO's)