Sergio Martinez knocked out Paul Williams with an amazing left hand hook.
Martinez' blow connected squarely with Williams' face and dropped him straight on the floor in the second round of their middleweight title match.
It was the 25th time that Martinez had knocked out an opponent. He improved his record to 46-2-2.
Martinez, 35, lost to Williams—the first time they faced each other—in May 2009. Their first match was a well-fought, 12-round battle which ended with a majority decision siding in Williams' favor.
Martinez immediately bounced back after that loss and won the World Middleweight Championship from Kelly Pavlik on April 17, 2010.
"I didn't want the judges to rob me this time," Martinez told reporters at the end of his fight.
Williams, 29, lost for only the second time in his career. His overall record dropped to 39-2-0, with 27 KO's.
Of his knockout, Williams briefly reflected as he was attended to by paramedics, "I got caught with a punch."
The boxing match took place at the Boardwalk Hall of Atlantic City in New Jersey.
Information from the Associated Press was used in this report.