Mexican great Saul "Canelo" Alvarez knocked out Sergey Kovalev with a vicious combination to win the WBO light heavyweight title, becoming a four-division champion.
The 29-year-old middleweight king, stepping up two weight divisions, caught Russia's Kovalev with a left hook-right hand combination that sent the former champion almost crashing through the ropes near the end of the 11th round at the MGM Grand Garden Arena in Las Vegas.
Kovalev was officially knocked out at two minutes and 15 seconds of the 11th and stayed down for some time while being attended to by his corner and the doctor.
It was one of the most devastating knockouts of the brilliant career of middleweight king Canelo, who was fighting in the 175 pound division for the first time.
The fight had been close throughout with Kovalev winning most of the early rounds by using a steady jab to keep the burly Canelo at a distance.
Canelo became only the fourth boxer in history to win a junior middleweight world title and one at light heavyweight, a spread of 21 pounds. The other three are Sugar Ray Leonard, Thomas Hearns and Mike McCallum.
Despite giving up a size and reach advantage, Canelo's best punch throughout was the left hook which he used in the 11th to initially send Kovalev staggering backwards to the ropes. As a defenceless Kovalev was going down, Canelo then landed a vicious right hand to the face that knocked the champion out cold.
Canelo is now on a four-fight winning streak with his lone blemish being a majority draw against Gennady Golovkin in their first of two bouts.
He sought out Kovalev to add another belt to his impressive collection which includes titles at junior middleweight, middleweight and super middleweight.
Kovalev was the biggest name in the light heavyweight division having boxed there his entire career, which began in 2009.
The Russian was hoping that a win over pound-for-pound king Canelo would help resurrect an ageing career that had been on shaky ground of late.
Kovalev lost his WBA, IBF and WBO belts in back-to-back defeats to Andre Ward in 2016 and 2017, but regained a share of the title after beating Colombia's Eleider Alvarez in a rematch in February.
Kovalev was coming off an 11th-round knockout of mandatory challenger Anthony Yarde in August, making this one of the shortest breaks between fights in his career.
In a bizarre move for a major boxing card, the main event was delayed for more than an hour, with both fighters left to wait in their dressing rooms with gloves, on because the boxing streaming service DAZN wanted to wait until a televised Mixed Martial Arts bout in New York finished.
The Vegas boxing crowd was shown the martial arts fight on a big screen in the arena while they waited.