Badou Jack stopped Dervin Colina in the fourth round on the Floyd Mayweather Jr-Logan Paul undercard in Miami.
Jack scored a fourth-round knockout against Colina at Hard Rock Stadium on Sunday – the final fight before the exhibition showdown between boxing great Mayweather and YouTube sensation Paul.
Sweden's Jack (24-3-3), who is a former super middleweight and light heavyweight champion, dropped underdog Colina (15-1) three times in the fourth round.
"I'm a way better boxer than him," Jack said after Colina stepped in to fight on barely a week's notice, having replaced WBC light heavyweight champion Jean Pascal – who tested positive for four banned substances. "Outbox him, and everything else will come."
Earlier, Luis Arias (19-2-1) claimed a shock split-decision victory over Jarrett Hurd (24-2) in the 10-round middleweight contest.
"People close to me were telling to get a job, give up on boxing, and I'm here," Arias said.
"I believe I won the fight," Hurd said. "I had him out sometimes and they gave him a break wiping off the ring, but they gave him the fight and I have to go with what they said.
"The plan was to come in here and box but we couldn't move around as much because the ring was slippery."