NEW YORK – Some of the best movies don't finish with happy endings — such is the story for Miguel Cotto's boxing career on Saturday night.

Three years after stopping heavily favored Luis Abregu in Atlantic City, the Brooklyn native ruined Miguel Cotto’s farewell fight by upsetting the Puerto Rican legend in their 154-pound championship match at Madison Square Garden. Ali, who moved up from welterweight to super welterweight to challenge Cotto, hurt Cotto in the second and fourth rounds, came back from some trouble in the middle rounds and won the final four rounds on all three scorecards to earn a unanimous decision.

The 29-year-old Ali (26-1, 14 KOs), a 2008 Olympian, won the WBO super welterweight title from Cotto (41-6, 33 KOs) in his first fight as a full-fledged 154-pounder.

All three judges – Julie Lederman (115-113), Eric Marlinski (116-112) and Steve Weisfeld (115-113) – scored the 12-round fight for Ali. The 37-year-old Cotto went off as an 11-1 favorite and was widely expected to stop Ali, who lost by technical knockout to light-punching former welterweight champion Jessie Vargas in their March 2016 title bout.

It was a fairly even fight — a few rounds in the middle could have gone either way. Ultimately, Sadam Ali took over the fight and it was enough on the judge's scorecards, winning the fight via unanimous decision.

Cotto revealed — and it was apparent — that he tore his bicep in the seventh round, as the muscle was rolled up into his upper arm. Cotto, all class, refused to use the injury as an excuse for his loss. 

He wanted to face a more appealing opponent in his farewell fight. Several high-profile fighters – most notably IBF welterweight champion Errol Spence Jr. (22-0, 19 KOs) and WBC lightweight champion Mikey Garcia (37-0, 30 KOs) – turned down the opportunity to challenge Cotto, though, because they didn’t want to sign long-term contracts with Golden Boy Promotions, Cotto’s promoter, to get the fight.

Ali, also promoted by Golden Boy, jumped at the opportunity. Though a career welterweight, Ali was confident he could upset Cotto to jumpstart his stagnant career.

The last time he was a big underdog, Ali stopped the hard-hitting Abregu in the ninth round on the Sergey Kovalev-Bernard Hopkins undercard three years ago at Boardwalk Hall in Atlantic City. He lost to Vargas thereafter, but more than atoned for that defeat Saturday night.

“Good things happen to good people,” Ali said. “I have been training since I was 8 years old, and I am glad I got this win at MSG, in my hometown.”