Ryan Garcia floored Devin Haney three times on the way to a majority decision victory in their 12-round super-lightweight fight in New York City, with the 25-year-old defying his concerning antics before the fight.

Garcia spewed a series of erratic social media posts and shared several conspiracy theories in the weeks leading up to the fight with WBC super-lightweight world champion Haney.

He said he had somehow lost access to his credit cards and mobile phone, sparking concerns for his wellbeing, although he assured followers he was OK.

But Garcia showed his rapid hands and all-round boxing prowess once he got in the ring, laying out Haney with a trademark left hook in the seventh round, and sending the previously undefeated fighter to the canvas two more times, in the 10th and 11th rounds.

"Come on, guys, you really thought I was crazy? You lost your whole mind," Garcia said after his victory.

"My left hook is my left hook. That's blessed by God."

Judges scored the fight 112-112, 114-110, 115-109 in favour of Garcia, though the American was ineligible for Haney's world title after missing weight.

Garcia skolled from a beer stubby on the scales at the pre-fight weigh-in, although he later said it was sparkling apple juice.

Regardless, Garcia ended up 3.2 pounds (1.45 kilograms) over the 140-pound weight limit, with Haney saying Garcia honoured a bet to pay him $US1.5 million  — $US500,000 for every pound he was over.

"I don't give a f*** what people say about me," Garcia said in his post-fight press conference, alongside boxing legend Oscar de la Hoya.

"Not necessarily am I proud of that, but I'm just saying, it was a statement to show you guys can't really f*** with me."