Australia’s George Kambosos Jr won the IBF, WBA and WBO lightweight titles from Teófimo López in a seismic upset on Saturday night at the Theater at Madison Square Garden.

The unheralded Sydney native, who went off as a 6-1 underdog, dropped López in the opening round, then came off the floor himself in the 10th to win a split decision before a rollicking crowd of several thousand spectators almost entirely in the champion’s corner. Two of the ringside judges handed down scores 115-112 and 115-111 for Kambosos while the third scored it 114-113 for López. (The Guardian had it 114-112 to Kambosos.)

The gripping encounter was the upset of the year and, quite possibly, the fight of the year.

“I believed in myself, I backed myself,” Kambosos said in the immediate aftermath. “I said it time after time: You might not believe it, but I believe in myself. And look at me now. I’ve got all the jewels. I’m not the king, I’m the emperor because I come to every other country and I take them out one by one.”

López, the 24-year-old Brooklyn native and one of the sport’s brightest young talents, was back in action for the first time since delivering on his enormous promise more than 13 months ago with a comprehensive unanimous-decision win over Vasiliy Lomachenko, the three-weight champion from Ukraine who for years had been widely regarded as boxing’s pound-for-pound best.

That more than four hundred days passed between López’s star turn in the MGM Grand bubble and Saturday night’s thrice-postponed date with Kambosos, the IBF’s mandatory challenger, staged in the Garden’s smaller theater instead of the big room next door, offers an instructive if mind-numbing case study in boxing’s tragicomic institutional dysfunction.

But Kambosos (20-0, 10 KOs), an unbeaten 28-year-old who earned his crack at the belts with a career-best win over the Welshman Lee Selby last year, made the most of the delayed opportunity, enduring López’s smothering onslaught in the early rounds, scoring a knockdown against the run of play and rallying from near-defeat to deliver the finishing kick of a champion in his opponent’s hometown.

López (16-1, 12 KOs) had said that patience was the big lesson that he learned during his tumultuous year on the shelf, but his pell-mell approach from the opening bell as he swarmed Kambosos with loaded punches behind a menacing glare in search of a first-round knockout was the embodiment of haste and recklessness.

The challenger fought deftly off the back foot as he weathered the hail from his heavy-handed foe, looking for opportunities to counter while trying to find his rhythm. Then near the end of the opening frame, Kambosos sent López crashing to the canvas with a flush counter right over the top that the champion never saw coming. López beat the count, but ate another concussive right from the Aussie right before the bell. The fight was on.

“When Muhammad Ali fought an unbelievable puncher in Joe Frazier, [he] rang Cus D’Amato and said, ‘What do I do?’” Kambosos said. “And [D’Amato] said, ‘You hit him with the best right hand you’ve ever thrown in your life in the first round and you change the fight.’ And that’s what I did. I had that in my head. That was my mentality: ‘I’m going to hit this guy clean, I’m going to him hard and I’m going to put him down.’ And the fight changed after that.”

López spent the early rounds largely repeating the failed formula of the first, rashly flailing into the pocket with looping shots that Kambosos either absorbed well or slipped entirely. Kambosos continued to score with well-timed counters, most notably with blind overhand rights. By the end of the fourth both fighters were throwing together in the center of the ring and it appeared Kambosos was getting the better of the exchanges, just.

The champion was already far behind on all three judges’ scorecards with swelling under both eyes by the middle rounds when he realized he needed to impose his size and physicality with greater measure and ring intelligence. Kambosos enjoyed his best round of the fight in the sixth, boxing off the back foot beautifully, countering with brio and opening a cut over López’s left eye that would trouble him through the rest of the fight.

By the eighth, Kambosos was beating López to the punch consistently when López, whose once-frenetic work rate had started to wane, bothered to throw at all. But even this reduced version of the champion remained dangerous in those moments he barged into the pocket throwing punches with cruel intentions. He finally seemed to wrest back the momentum of the contest in the ninth, where he landed a three-punch combination with a minute to go that moved Kambosos backward and whipped the hometown crowd into a frenzy. López finished the round unloading on the Aussie along the ropes only for Kambosos to escape danger when the bell sounded.

Then came the 10th, when López dumped the bloodied challenger to the floor early in the round. Kambosos beat the count but appeared in serious trouble, scarcely throwing back and bleeding badly from above his left eye as López doggedly moved in to close the show. Perhaps one clean shot from defeat with the referee Harvey Dock prepared to intervene at any moment, Kambosos somehow made it to the bell upright and the fight pressed on into the championship rounds.

“I tried to entertain the crowd a little bit too much,” Kambosos said. “Got excited a little bit too much and I got caught. But you know what? What a warrior. I got back up, against all odds, and still finished the fight and won the next two rounds.”

Kambosos’s recuperative powers saw him bounce back remarkably in the 11th, peppering López with jabs and straight punches and while opening the cut over López’s left eye until it poured blood down his chest and on to his trunks. Sensing he needed something special in the final round, López went for broke over the final three-minute session but Kambosos continued to counter sharply while staying out of harm’s way.

“All respect to Teofimo,” said Kambosos, who connected on 182 of 739 punches (24.6%) compared to 176 of 565 for López (31.2%), according to Compubox’s punch statistics. “He’s a great kid. The buildup was the buildup. We’re both competitive, young guys. But it was my night and it’s going to be my night for a very long time.”

So exhaustive was the Aussie’s performance that López was booed lustily by the same fans who had cheered him all night when he interrupted Kambosos’s in-ring interview to claim he won the fight.