Glen Johnson earned a split decision over Antonio Tarver in a tense IBO light heavyweight bout in Los Angeles.
Johnson was the aggressor from the first bell, landing several strong punches while Tarver vainly sought the big left that beat Roy Jones Jr in May.
Tarver fought back well but a good show from Johnson in the last three rounds swayed the judges in his favour.
Johnson had surrendered the IBF belt and Tarver the WBC title so that they could fight each other.
After the fight, Jamaica's Johnson refused to claim he was the supreme light heavyweight in the world.
"I'm still looking for Mr Best. I would never say that I was the best," he said.
Johnson added that his opponent had never managed to hurt him.
"He knocked me off balance a couple of times, but I was never hurt," Johnson said.
"It was a real close fight. I'd like to fight him again."