American Ryan Garcia floored Devin Haney three times in a majority points win over his compatriot on a dramatic night in Brooklyn, New York.
Garcia’s left hook was his greatest asset as the previously unbeaten world champion Haney was floored in the seventh, 10th and 11th rounds.
Scorecards of 112-112, 114-110 and 115-109 saw the decision go to Garcia.
The Californian, however, did not win Haney’s WBC light-welterweight title after missing weight for the contest.
“You really thought I was crazy? You all lost your own minds,” said Ryan Garcia, who had been criticised for his behaviour in the build-up.
The 25-year-old had posted a series of erratic, often unsubstantiated and defamatory social media posts in recent months.
“You guys overegg everything,” Ryan Garcia added.
He extended his record to 25 wins and one defeat, while it was Haney’s first pro defeat in 32 bouts.
In his post-fight news conference, Ryan Garcia said he had been “drinking every night” in the build-up to the bout after getting divorced earlier this year.
“Not necessarily am I proud of that, but I’m just saying – I do whatever I want and still win,” he added.
A marked-up Haney, 25, said he was “disappointed” in his performance.
He added: “I [showed I] was a true champion and I could fight after being knocked down.”
On the undercard, Belfast’s Sean McComb was controversially defeated by light-welterweight Arnold Barboza Jr.
In a world-title eliminator, McComb produced a composed performance with slick footwork and better timing but lost a split decision.
Underdog Garcia steals the show in Brooklyn
The pair had shared the ring six times as amateurs, with three wins each, but it was underdog Garcia – who missed the weight by 3.2lb – who stole the show in the pro ranks at the Barclay Center.
He made a blistering start, landing a lead left hook in the first minute as Haney’s legs buckled.
‘The Dream’ recovered well and began to take control, glancing a left hook of his own in the third as American greats Mike Tyson, Evander Holyfield and Roy Jones Jr watched on from ringside.
In a chaotic and eventful seventh, Haney – while his father and trainer Bill was being interviewed in the corner by the TV broadcaster – hit the canvas for the first time as a pro.
Haney rose to his feet, albeit on shaky legs. An overexuberant Garcia – who had a string quartet perform classical music in his dressing room earlier – was deducted a point seconds later for throwing a punch after the referee asked the boxers to break.
Garcia continued to attack as Haney, holding on, slumped to the canvas twice more later in the round but both were ruled as non-knockdowns.
Just as Haney appeared to regain control, landing solid body shots, Garcia floored the champion in the 10th with his quick hand speed. Haney was hurt again in the final 30 seconds by another Garcia hook.
The sight of Haney on the canvas was becoming all too familiar. He was open for the left hook and Garcia obliged in the 11th.
“I fell asleep on the left hook. We trained for it but I got in there and fell asleep,” said Haney, who has also been undisputed champion at lightweight.
“I gave him a shot, it’s only right he gives me a shot back. He didn’t make weight, so I’m still the champion. We can run it back.”