The suspicions that Canelo Álvarez’s pay-per-view showdown with Julio César Chávez Jr was a sideshow not worthy of your time, say nothing of the hefty $70 price tag, proved founded early in Saturday’s one-sided beatdown on the Las Vegas Strip. This one was more of an event than a fight: a celebration of Cinco de Mayo featuring two of Mexico’s most popular boxers masquerading as authentic sports experience for fans.

Álvarez won every minute of every round against an opponent who is his rival in narrative only. Not once during the 12-round mismatch, which took place at a contracted catchweight of 164½lbs, did the popular red-headed puncher from Guadalajara sit on his stool between rounds. One could imagine Canelo suffered sparring sessions in training camp more taxing than Saturday’s light workout before a sold-out crowd at the T-Mobile Arena. When it was over, all three ringside judges turned in identical scores of 120-108, as did the Guardian. A total whitewash and, candidly, a bit of an embarrassment.

That only set the stage for the night’s real news. Afterward, Álvarez announced he will face Gennady Golovkin, the charismatic Kazakh knockout artist known as Triple G, in a long-awaited superfight to unify the middleweight championship. Pen was put to paper last week and no venue has been set. But the biggest fight in the sport that can be made today has been signed and it will happen on 16 September at the division limit of 160lbs.

“GGG, you are next my friend,” the 26-year-old Álvarez, a former champion at junior middleweight and middleweight and the presumptive face of the sport in boxing’s post-Mayweather age, said through a translator. “The fight is done. I’ve never feared anyone, since I was 16 fighting as a professional. When I was born, fear was gone. I never got my share of fear. I’m very happy, and the rivalry is going to show my skills even more. I’ve had difficult fights, and that will no doubt be a tough fight. But, I always say, Canelo Álvarez is the best because I fight the best.”

The shrewdly timed announcement, an inspired piece of stagecraft which speaks volumes about what Canelo’s handlers thought of Chávez’s chances, was a convenient way to make people forget about what they had just watched.

Canelo (49-1-1, 34 KOs), whose lone career defeat was a majority-decision loss to Floyd Mayweather in 2013, was busier and landed more frequently from the opening round as the almost comically bigger Chávez failed to leverage his formidable advantages in size and reach.

Álvarez lands a thudding body shot during Saturday’s fight. Photograph: ddp USA/REX/Shutterstock

Chávez (50-3-1, 32 KOs), a former middleweight beltholder who lost his strap to the great Sergio Martínez five years ago, was so drained at Friday’s weigh-in that he looked ready to pass out, prompting fears he might come out even slower due to the weight cut. Those fears, it turned out, had merit. The son of the greatest fighter in Mexico’s rich fistic history labored to produce anything resembling an offense and Canelo, who dictated the distance, standing mid-range unmolested in what felt like acres of space, got off first time and again. By the end of the third round, a trickle of blood ran from the bigger man’s nose.

By the sixth, Canelo was simply walking Chávez down, setting up his punches beautifully, unconcerned with single-shot return fire, landing as many per round as his opponent was throwing. Even when Chávez had openings to uncork, he couldn’t pull the trigger. Thus he appeared content to lay back in an effort to extend the fight, perhaps endowed with the grim self-knowledge that any resistance would only hasten his demise.

By the ninth, Chávez’s left eye was nearly fully closed and a feeling of discomfort seemed to prevail among the well-lubricated capacity crowd of 20,516, who had started the night bellowing singalongs in the stands to Mexican standards Tragos Amargos and La Ley Del Monte but had now descended into cascades of boos. For what exactly? Certainly not the surgical work of Álvarez, who gave no quarter in the so-called championship rounds in his effort to close the show. The bloodletting was on and it was hard to understand how anyone could feel particularly good about bearing witness to the punishment, which truly should have been stopped by referee Kenny Bayless or any sane party. For years Chávez’s commitment has been called into question, and correctly, but no one can doubt his chin: even from a smaller man these were devastatingly hard shots.

The 31-year-old Chávez, who earned a guaranteed $3m for his trouble (compared to $5m for Álvarez), managed to finish the bout upright but nothing else. Few at ringside could remember the last time a fight put forth as a top-flight contest had ended with all three ringside judges failing to find even one round to give the loser.

“Tonight I showed I could move, I could box, I showed as a fighter I can do all things,” said Álvarez, who landed 228 of 604 punches (37.7%), compared to 71 of 302 for Chávez (23.5%). “I thought I was going to showcase myself as a fighter that could throw punches, but he just wouldn’t do it. I’ve shown I can do lots of things in the ring, anything a fighter brings. I’ve shown I can showcase myself.

“I wanted to try something new. I never sit down in sparring and I didn’t want to sit here.”

Said Chavez, also through a translator: “I wanted to box but he went to the ropes and I just needed to throw more punches. I would’ve attacked more I would’ve been countered by his punches. [Trainer Nacho Beristain] told me to do that but the strategy didn’t work.”

Now the eyes of the boxing world turn to the hotly anticipated meeting between Canelo and Golovkin, who watched from ringside on Saturday night.

“The fight is signed, sealed and delivered,” said Oscar De La Hoya, who promotes Álvarez. “I’ve already had several calls from people interested in staging this fight. I have a missed called from Dubai. I have a missed call from the UK where Anthony Joshua and (Wladimir) Klitschko just sold out 90,000 people. There’s interest from all over the world. There’s interest from everywhere.”’

Álvarez poses with September opponent Gennady Golovkin after Saturday’a fight. Photograph: Erik Verduzco/AP

De La Hoya said the fight was “not tough at all” to make, adding that negotiations began several months ago but were halted out of respect for the Chavez fight. The Golden Boy Promotions founder said they restarted negotiations a few weeks ago and the contract was signed this week.

That set the stage for Saturday’s dramatic rollout. As Canelo conducted an in-ring interview with HBO’s Max Kellerman after the fight, the lights went down immediately when he announced Golovkin would be his next opponent. A video teaser played on the big screen as the Kazakh emerged from the tunnel to the ominous seven-note guitar lick of the White Stripes’ Seven Nation Army, his traditional ringwalk song.

“I feel very excited, right now is a different story,” Golovkin said. “In September, it will be a different style. A big drama show. I’m ready. Tonight, first congrats to Canelo and his team. Right now, I think everyone is excited for September. Canelo looked very good tonight, and 100% the is the biggest challenge of my career. Good luck to Canelo in September.”

Golovkin, 35, is coming off a points win over Daniel Jacobs at Madison Square Garden two months ago, his 18th straight middleweight title defense. It marked the first time in 24 fights he’d been extended the full distance, snapping a near-decade-long knockout streak that extended back to when he was fighting eight-rounders during the George W Bush administration. In that span he’s absorbed three of the four major world title belts at middleweight, while Canelo is lineal champion by virtue of his career-best win over Miguel Cotto in November 2015 (though, notably, that bout took place at a catchweight of 155lbs, five below the division limit).

A crowd-pleasing puncher, Golovkin has long been regarded as boxing’s most feared and avoided combatant, perhaps too good to get a big fight. No longer. September’s blockbuster will be as big as it gets: the most significant event the sport has known since 2015’s summit meeting between Floyd Mayweather Jr and Manny Pacquiao.

“Everyone understands this is a tough fight for both of us,” Golovkin (37-0, 33 KOs) said afterward. “This is amazing fight for everyone. Of course, I respect his team. Everybody wanted this fight.”