Answer
The former world champion he beat was Lamon Brewster. Brewster was a former heavyweight world champion who had held the WBO heavyweight title from 2004 to 2006. He had also challenged for the WBC heavyweight title in 2005, but was defeated by Vitali Klitschko. In his fight against Lewis, Brewster was knocked down in the first round and again in the second round before the fight was stopped.