Klitschko, who took the fight on two weeks notice when Kirk Johnson pulled out with an injury, spent the first two rounds having his way with an overweight, overconfident Lewis, who may have taken the Ukrainian challenger a bit too lightly.
But the idea of a huge upset was dashed in the third round, when the Brit scored with a pair of long right hands, one of which opened a nasty cut across Klitschko's left eyelid.
The challenger pressed on, but the cut became worse. Despite fervent corner work, the entire left side of Klitschko's head was caked with blood when the sixth round came to an end.
Dr. Paul Wallace looked at the inch-long gash and recommended that referee Lou Moret stop the fight.