One of the most unusual build-ups to a major fight in recent memory delivered an even more sensational outcome on Saturday night when Ryan Garcia scored three knockdowns of Devin Haney and handed the WBC super lightweight champion the first defeat of his professional career in a dramatic upset.

Related: Ryan Garcia wins stunning majority decision over Devin Haney – as it happened

Garcia, who went off as a 6-1 underdog, dropped Haney in the seventh, 10th and 11th rounds behind an explosive cocktail of speed and power to win a majority decision by scores of 115-109, 114-110 and 112-112. (The Guardian had it 113-111 to Garcia.)

Their scheduled 12-round meeting at Brooklyn’s Barclays Center had been downgraded to a non-title bout on Friday when Garcia weighed in at 143.2lbs, a yawning 3.2lbs above the division limit and a debacle that was interpreted as a sign of ill preparation and tumult within his camp. It also created a situation where Garcia could no longer take Haney’s championship, only his unblemished professional record. Against all odds that’s exactly what happened.

On paper it was the caliber of event that’s all too rare in boxing today: a matchup between two of America’s brightest young stars, both 25 years old and at the front of their athletic primes with ample top-flight experience. But it was almost entirely overshadowed by Garcia’s erratic behavior both in person and on social media, which descended into a disturbing blur of conspiracy theories and apparent cries for help.

“Come on, guys, you really thought I was crazy?” Garcia said afterward.

Garcia came out guns blazing from the opening bell, wobbling Haney with a left hook that sent the crowd into hysterics and prompted chants of “Ry-an! Ry-an!” from the upper reaches of the arena. Haney took a more aggressive tack over the next few rounds and began dutifully working behind the jab, backing the challenger up, rocking him with a right hand in the third and visibly frustrating Garcia as he ate one counter after another while pressing for openings to land his prodigious left hand.

But just when it looked as if Haney was pulling away on the scorecards, Garcia detonated a left hook early in the seventh that sent the champion to the floor for the first time in 32 professional fights. Garcia was slapped with a point deduction by referee Harvey Dock moments later for hitting on the break while trying to close the show, giving Haney precious seconds of recovery time, but the badly hurt champion appeared on rubbery legs well into the eighth.

Haney showed remarkable grit and recuperative powers but couldn’t keep out of the way of Garcia’s loaded shots. After going down beneath a heavy right in the 10th and a vicious counter left in the 11th, the bloodied champion spent most of the final session on the receiving end of Garcia’s taunts. Seldom has boxing’s reputation as the theater of the unexpected been more apt.

“I’m disappointed with my performance,” Haney said. “But I showed I’m a true champion and I can fight after being knocked down and hurt. He caught me early, caught me by surprise. We trained for (the left hook), but I got in there and I fell asleep, and he caught me with it.

“I was more surprised than hurt the first time. He jumped on me, like we knew he would, but I was just sleeping. I thought the ref let him turn his back and hold a little too much.”

Garcia landed 87 of 214 blows (40.7%) according to Compubox’s punch statistics, compared to 106 of 285 for Haney (37.2%). But the disparity in power shots made all the difference with Garcia landing more than twice as many (95) as his opponent (45).

The promotion had largely been marketed around their rivalry in the amateurs, six bouts in all with each fighter winning three. But while Garcia may be the bigger celebrity today – with upwards of 10.5m Instagram followers with 7.5m more on TikTok – there’s no question Haney came in as the more accomplished professional and the overwhelming favorite on merit.

The Bay Area native had unified all four major titles at 135lbs with a career-best win over Vasiliy Lomachenko in May, before climbing to 140lbs and becoming a two-weight champion with an impressive shutout of Regis Prograis to win the World Boxing Council’s version of the super lightweight title. (That belt was thought to be vacated with Saturday’s result, although WBC president Mauricio Sulaiman said afterward that Haney would remain the organization’s champion.)

Over the same timespan Garcia’s status as boxing’s fresh-faced superstar-in-waiting had been greatly diminished. He suffered his first career defeat to Gervonta Davis last year, getting stopped in the seventh round by a crunching body shot with a seeming lack of urgency to beat the count that some critics decried as a business decision. And while he rebounded admirably with an eighth-round stoppage of Oscar Duarte in December, Garcia’s highly volatile comportment over the past few months had left onlookers terribly pessimistic about his chances against an opponent who has steadily been climbing boxing’s pound-for-pound charts.

When it became evident in Saturday’s aftermath that Garcia had fooled them all, the Orange County native didn’t shrink from taking a victory lap.

“This is why people need to stop believing everything on the internet and stop living in a false reality,” Garcia said. “At the end of the day there’s a lot of real shit going on. And the last thing you should worry about is a kid acting crazy on the internet.”

“I put my reputation on the line and had everyone thinking I was crazy. At the end of the day, who’s the crazy one now?”

Both Garcia and Haney expressed interest in a rematch, a no-brainer proposition after Saturday’s wildly entertaining scrap. But Oscar De La Hoya, who promotes Garcia, made sure to underscore the new world order.

“Ryan is going to rest,” De La Hoya said. “(He) had a great win, let him enjoy it. But guess what: he’s calling all the shots now. That’s the fact. Whether he wants to fight at [147lbs] or at a catch-weight, if anyone wants to make money they have to fight Ryan. And he’s going to call all the shots.”