Saul 'Canelo' Alvarez unified the middleweight division with a points victory over Daniel Jacobs in Las Vegas.
The Mexican - now beaten just once in 55 fights - took Jacobs' IBF belt to add to his own WBC and WBA titles.
Judges scored the fight 115-113, 115-113, 116-112 in Canelo's favour after 12 rounds.
And the scorecard reflected Canelo's improved defence and counterattacking skills in his return to 160lb.
The 28-year-old became a three-weight world champion when moving up a division to face England's Rocky Fielding last year, but he returned to middleweight to face New Yorker Jacobs, 32, in what was his second bout of a reported £278m deal with streaming service DAZN.
Canelo was busy in the early rounds but Jacobs grew into the fight and landed a heavy left hook in the ninth which the Mexican's chin stood up to.
"He's a tremendous champion, I take my hat off to him," said Jacobs - known as the 'Miracle Man' after returning to the ring and becoming a champion following cancer.
"I definitely think it took me a couple of rounds to get my wits about me. I feel like I gave enough to get the victory, but I've got to go back to the tapes and see what happened."
Boxing fans will now be hoping to see Canelo face another American - WBO middleweight champion Demetrius Andrade - to become the undisputed middleweight king.
"Nobody in Mexico has ever done that, won all four belts in one division, so that's my motivation," Alvarez said, before the Jacobs fight.
However, after defeating Jacobs, Canelo said he was open to meeting Kazakh fighter Gennedy Golovkin for a third time.
The two drew their first bout in 2017, then Canelo took a controversial points victory in September 2018.
"If he wants to do it again, I'll beat him again," he said.
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