The hastily put together fight was held in front of 61,000 fans at Veltins Arena in Gelsenkirchen, the biggest boxing audience in Germany in 70 years, according to gmanews.tv.
Klitschko relied on a powerful left-right combination to send Chagaev to the mat in the second round, and exploited a cut that opened over the Uzbekistan-born fighter's left eye in the eighth.
Referee Eddie Cotton called the fight for Klitschko at the opening of the 10th round.
The Ukrainian's 5-inch (13-centimeter) height advantage and superior power helped him wear Chagaev down from a safe distance, though Chagaev maneuvered inside Klitschko's reach for several early blows that appeared to stun Klitschko, and one that raised a large bruise under his right eye.
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