Eleider Alvarez knocked out Sergey Kovalev in the seventh round on Saturday with a stunning upset victory to capture the World Boxing Organization light heavyweight title.
Alvarez, of Colombia, hit Kovalev with a devastating right hook that sent the Russian to the canvas for the third and final time late in the seventh round.
The 35-year-old Kovalev was trying to re-establish his dominance in the light heavyweight division in his first real boxing test since losing in back-to-back fights to Andre Ward.
Kovalev was the more aggressive of the two boxers and was winning the majority of the rounds heading into the final round at the Hard Rock Hotel in Atlantic City, New Jersey.
But he let down his guard in the seventh and Alvarez seized the opportunity to land several power punches and go on to win the fight in one of the biggest boxing upsets of the year.
He knocked Kovalev down twice and then finished him off with consecutive right hooks to the head that sent him down for the third and final time. At that point the referee moved in and called an end to the fight with 15 seconds left in the round.
Kovalev, who won his last two fights over a pair of weak opponents, dropped to 32-3-1 with 28 knockouts.
Alvarez, who is based in Montreal, Canada, improves to 24-0 with 12 knockouts.
The result derails plans for a light heavyweight unification bout between Kovalev and Dmitry Bivol, who retained his World Boxing Association belt with a unanimous decision over Isaac Chilemba on the same card.
Bivol controlled the early and middle rounds but seemed to tire near the end as Chilemba started to win some rounds.
One judge scored the fight 116-112 for Bivol, while the other two had it 120-108.