Unbeaten Danny Garcia stopped Erik Morales with a powerful left hook in the fourth round to retain the World Boxing Association and World Boxing Council light welterweight titles.
Garcia's punishing left hand spun Morales and sent him falling onto the bottom rope of the ring on his back.
When a Morales cornerman climbed into the ring, referee Benji Esteves stopped the fight 83 seconds into the fourth round.
"He's a crafty veteran," Garcia said. "I wanted to set up my power punches and give him a little extra. I thought this fight was going the distance. I'm glad to have it done early."
Garcia improved to 25-0 with his 16th early stoppage, dropping Morales to 52-9, and thanked his mom for the winning punch.
"I get that from my mom," he said. "All my family is left-handed."
Garcia beat Morales by unanimous decision in March, when Morales was overweight at the weigh-in and unable to win the crown, and stopped former champion Amir Khan of Britain in the fourth round in July.
A Khan rematch is among the options for Garcia's next fight.
"I won't duck anybody," Garcia said. "I've got the titles. If you want them come get them. But it ain't going to be easy."
Garcia, 24, considered not climbing into the ring after Morales was found to have tested positive for the banned steroid clenbuterol in a random drug test.
Morales, 36, blamed the result on tainted food in his homeland that had produced positive results on other athletes, notably football players ahead of World Cup qualifying matches.
"I was definitely calm about this. I did nothing illegal," Morales said. "I was calm because I have done nothing wrong."
But Garcia said he was fine with carrying on after New York state officials sanctioned the bout.
The fight was the main event in the first major fight card in Brooklyn since 1931 and it came at the new $1 billion Barclays Center, the new home of the NBA Brooklyn Nets, who moved from New Jersey after last season.
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