Oleksandr Usyk turned his sights on rival heavyweight world champion Tyson Fury on Sunday after beating Britain's Anthony Joshua on a split points decision to retain his WBA, WBO, IBF and IBO belts in Jeddah.
The "Rage on the Red Sea" in Saudi Arabia was a rematch of a fight in London in September, which the Ukrainian won on a unanimous decision to take Joshua's belts, but with more intensity and emotion.
The pressure was on both former Olympic champions, with Usyk this time representing a country fighting for its existence after a Russian invasion while Joshua was battling for his boxing future.
Usyk appeared comfortably ahead as the final bell rang at the King Abdullah Sports City Arena but the American judge surprisingly awarded the fight 115-113 to Joshua.
The British and Ukrainian judges decided 115-113 and 116-112 to Usyk.
The victory took the 35-year-old's professional record to 20 fights undefeated while Joshua, 32, suffered a third defeat in what could prove a watershed in his career.
Joshua, who had held aloft the Ukrainian flag with Usyk as they waited for the decision in what seemed an acceptance of defeat, then had an uncharacteristic meltdown.
He took two of the belts, dropping them as he left the ring and headed for the dressing room before turning around and stepping back between the ropes to take the microphone and address the crowd.
"Usyk is one hell of a fighter. That’s just emotion," declared the Briton.
"For this guy to beat me tonight, maybe I could have done better but it shows the levels of hard work he must have put in, so please give him a round of applause as our heavyweight champion of the world.
"I was studying Ukraine and all the champions from your amazing country. I’ve never been there. What’s happening there, I don’t know but it’s not nice ... under those circumstances he's managed to become champion."
The opening round was tentative, with Joshua's corner calling for him to adjust his rhythm, and the bout continued with Usyk using his jab and body shots to good effect.
Round eight was briefly halted for towels to mop the slippery, sweat-soaked canvas but round nine raised the heat again, with Joshua enjoying his best before Usyk came back hard in the 10th.
Needing a knockout and running out of time, the taller and heavier Joshua was unable to land the telling blows against an agile and elusive opponent who hit back hard.
"In the ninth round I thought we had him," said Joshua's promoter Eddie Hearn. "The 10th round was one of the best rounds I've seen.
"What Usyk did in the 10th, the 11th and the 12th was incredible and that was the difference tonight because AJ didn't start fast enough but he fought a much better fight.
"He hurt Usyk badly in the ninth and I felt like he was going to come on strong. But Usyk came out like a train."
At the final bell the two embraced and the Ukrainian, who had entered the ring in a yellow and blue "Colors of Freedom" top while Joshua was dressed all in black, dropped to his knees.
Usyk's thoughts then turned to the next chapter and another Briton to beat, although WBC champion Fury has declared himself retired.
"I am convinced he wants to fight me. I want to fight him. And if I'm not fighting Tyson Fury, I'm not fighting at all," Usyk told the crowd through a ringside interpreter.
Ukrainian President Volodymyr Zelenskiy hailed a "difficult but important and necessary victory."
"Defending a world championship title is a symbol that whoever is from the Cossack tradition will not give up what is his, will fight for it and will win without any doubt," he wrote on Facebook.