George Kambosos Jr stunned Teofimo Lopez to become the unified world lightweight champion on a split decision in Madison Square Garden.
The Australian, 28, won 115-111 113-114 115-112 on the judges' cards to take home the WBA, WBO, and IBF belts.
American Lopez, 24, was knocked down in the first round while Kambosos was sent to the canvas in the 10th in New York.
"I believed in myself, I backed myself," said Kambosos, who like Lopez was left bloodied and bruised.
Kambosos, who has now won his last three fights by split decision, added: "I thought, 'I'm going to hit him hard and put him down', and the fight changed off that. I'm an unbelievable boxer. I can't believe how good I boxed."
Before the fight Lopez had promised a first-round knockout, but it almost came for Kambosos when he caught the champion with an overhand right.
He maintained pressure but Lopez showed his class in what was an incredibly difficult fight to call.
Kambosos connected with another big right hand in the seventh, before an uppercut caught Lopez off guard.
With two rounds to go the challenger fell to an impressive combination from Lopez that sent him to the canvas, but Kambosos rallied to claim a shock victory.
Lopez was incensed by the result, saying: "I don't care what anybody says, man. I won this fight."
Lopez, who suffered his first loss in 17 fights, dethroned pound-for-pound great Vasiliy Lomachenko to add the WBA (Super) and WBO titles to his IBF belt 13 months ago in Las Vegas.
His defence has been much delayed because of issues with promoters and he had to pull out of a June fight date after contracting Covid-19.
Kambosos is now unbeaten in 20 fights and has already been called out by American Devin Haney, who holds the WBC lightweight belt.