The Mexican superstar Saul ‘Canelo’ Álvarez scored the signature win of his career in Saturday night’s long-awaited rematch with Gennady Golovkin, capturing the WBC and WBA middleweight titles by a majority decision in a classic encounter that all but guarantees a third installment in May.

Two of the finest pure fighters of their generation treated a sellout crowd of 21,965 at the T-Mobile Arena to a contest of extreme physical and psychological intensity that managed even to surpass their electric first meeting last year, which ended in a widely derided split draw. This one was just as close and not entirely beyond dispute, merely flecked by controversy rather than defined by it. Ringside judges Dave Moretti and Steve Weisfeld scored it 115-113 to Álvarez, while Glenn Feldman had it 114-114. (The Guardian scored it 116-112 to Golovkin.)

Álvarez (50-1-2, 34 KOs), the popular red-haired puncher from Guadalajara, now adds Golovkin’s belts at 160lbs to the lineal middleweight title he earned by virtue of a 2015 win over Miguel Cotto.

“I showed my victory with facts,” the 29-year-old Álvarez said afterward through an interpreter. “He was the one who was backing up. I feel satisfied because I gave a great fight. It was a clear victory.”

For Golovkin (38-1-1, 34 KOs), the razor-thin verdict marked his first defeat in 40 professional fights, the first at any level since the 2005 amateur world championships, and ended his division-record streak of 20 consecutive middleweight title defenses on level terms with Bernard Hopkins, who incidentally is a minor stakeholder in Golden Boy Promotions, which promotes Álvarez. Afterward the 36-year-old from Kazakhstan, the sport’s longest tenured active champion no more, walked solemnly to the dressing room without remaining in the ring for the customary in-ring interview.

Their first clash, 364 days ago in the same building, had seen Golovkin spend the night methodically cutting off the ring with deft footwork while Álvarez fought off the ropes with varying results. But from Saturday night’s first bell the action took on a radically different geometry, playing out almost exclusively in the center of the ring with neither fighter’s back touching the edge even once until the middle rounds. Golovkin didn’t wait long to establish the jab as his weapon of choice, doubling and tripling it up early while refraining from the heavy-handed combinations that have been his calling card in a career which, until Saturday, saw him finish 34 of 39 fights inside the distance.

A highly tactical opening, with neither man backing down and so little to separate them, began to heat up in the second and third as the fighters started to open up and trade harder shots. Golovkin continued to score at will with the jab, but Álvarez’s varied attack appeared to move the Kazakh backward and keep him off balance while the Mexican’s educated combination punching laid bare the difference in hand speed.

Álvarez’s dedicated body work, which would come to yield dividends in the later rounds, was effective in stemming Golovkin’s offensive, yet the longtime middleweight champion absorbed his opponent’s best shots with little apparent effect. Both men were countering exquisitely in a series of skirmishes that took on the feel of bullet chess: this was expert-level work from two top operators.

A thudding uppercut by Golovkin rocked Álvarez early in the fourth, prompting a roar from the crowd. He followed it with three straight jabs, then made his opponent pay with a compact left followed by a right square in the stomach. Yet right when it seemed Golovkin was taking over, his work rate began to slow and Álvarez was right there to take advantage.

By the sixth and seventh Canelo was commanding the pace and geography of the two-way action even as Golovkin’s jab continued to score, negotiating inside with hooks to the body and combinations upstairs. They traded hellfire in the eighth, showing incredible punch resistance. But it was Golovkin who betrayed signs of fatigue near the end of the frame, breathing with his mouth open and stepping away from the exchanges with atypical consent. Álvarez moving backward landed a left hook flush on Golovkin’s jaw, followed by three more shots that whipped the partisan Mexican Independence Day audience into a frenzy. When the bell rang to signal the end of the eighth, Canelo appeared in command.

Golovkin lands a straight right on Álvarez during Saturday’s fight. Photograph: Joe Camporeale/USA Today Sports

By then a cut opened had above Álvarez’s eye and he appeared to flag early in the ninth, but Golovkin soldiered through his exhaustion. Back and forth it went into the 10th, when Golovkin moved Canelo backward with a four-punch combination then briefly stunned his opponent with a right hand. Golovkin was landing the cleaner punches and by the 11th was on the clear attack, evoking the seek-and-destroy modus operundi that had become his signature, but Álvarez held his ground and refused to back down, never allowing the older champion to land without proper recompense.

Both fighters entered the final round with the belief the outcome was in doubt – correctly, as Canelo led by a slight 105-104 margin on all three cards – but it was Golovkin who demonstrated the stronger finishing kick, striking with a left hook and an uppercut to open the frame and pouring on the punishment as the crowd roared chants of “Tri-ple G! Tri-ple G!”, for the first time on the night not shouted down by the Canelo faithful. When the bell sounded the fighters embraced, then returned to their corners to await the official scores.

(If there was a controversy, it’s that Moretti and Weisfeld found a way to award the outcome-swinging 12th, as clear as Golovkin round as there was, to Álvarez.)

“I’m not going to say who won tonight, because the victory belongs to Canelo according to the judges,” Golovkin said after receiving eight stitches for a 5cm cut along his right eye. “I thought it was a very good fight for the fans, and very exciting. I thought I fought better than he did.”

Golovkin threw and landed with greater frequency on the night, connecting with 234 of 879 punches (26.6%), compared to 202 of 622 for Álvarez (32.5%). But the resolve and adaptability of the younger, primer Álvarez was enough to carry the day, if only by the slimmest of margins.

“I can’t complain, that’s what we have the judges for,” Abel Sanchez, Golovkin’s trainer, said. “We had a great fight, the one we expected the first time around. I had it close going into the 12th round. We had good judges who saw it from different angles. I can’t complain about the decision, but it’s close enough to warrant a third fight. Canelo fought a great fight.”

When asked if he would consider a third meeting, Golovkin, now twice denied what could or should have been the defining moment of a prolific career spent largely outside of the mainstream spotlight, was to the point.

“Under the right conditions, yes,” he said.

Álvarez didn’t back down from a third fight, where he would almost surely enter the favorite after playing the underdog role in the first two.

“That was a great fight, but in the end, it was a victory for Mexico,” Alvarez said. “And again, it was an opportunity, and I want to shout out to my opponent, the best in the sport of boxing. I am a great fighter, and I showed it tonight.

“If the people want another round, I’ll do it again.”