His opponent, Paul Butler, was in survival mode the entire fight, ultra reluctant to throw a punch for fear of opening up his guard.
Butler knew what he was up against: not just any boxer, but the one who earned the moniker "The Monster" for his unbridled power, lightning-quick hands and uncanny use of angles.
Through the years as the Japanese star sliced through various weight classes with ease, Naoya Inoue's triumphs have transformed him into something approaching mythical status.
Butler, a solid fighter who happened to hold the final belt that stood between Inoue and the undisputed bantamweight championship, found out firsthand what makes Inoue so special.
Even though he was in high guard during the entirety of their December meeting, Inoue still found a way. That he was able to stop Butler wasn't so impressive. That Inoue (24-0, 21 KOs) was able to do so despite Butler's unwillingness to engage? Exceedingly rare. Fighters often lambast their opponents for "running" to explain why they couldn't deliver a knockout in a dull fight. No excuses needed with Inoue.
He exploded into the American boxing consciousness with a trio of knockouts over top-10 bantamweights in title fights from 2018 to '19 in Japan, streamed in the early morning hours stateside.
Now, Inoue, ESPN's No. 2 pound-for-pound boxer, is poised for his toughest challenge yet, another chance for his legend to grow. In his first fight at 122 pounds, Inoue will challenge the junior featherweight division's top boxer, Philadelphia's Stephen Fulton, for his WBC and WBO world titles on Tuesday in Tokyo (4:30 a.m. ET, ESPN+).
Fulton (21-0, 8 KOs) is a pound-for-pound talent, sitting just outside the top 10 and is naturally far bigger than Inoue (he won his first title at 122 pounds.) Inoue won his first title at 108 pounds, and so far, his talent has proven too difficult to be contained by size.
"I'm pushing the boundaries of my build, my limits," Inoue, 30, said on ESPN's "Camp Life." "I really don't know what kind of fight this is going to be. I'm just going to do everything I can to win. If I get the chance I'll go for a knockout and if it doesn't go that way, I'll just focus on keeping my boxing solid to get that win."
Just how good is "The Monster"? We take a look at what makes Inoue so special ahead of his showdown with Fulton.
'Inoue has great eyes'
Inoue rarely makes a mistake, even though he's so offensive-minded. During a three-fight string, his first bouts at 118 pounds, Inoue blitzed the competition. First, he scored an eye-opening first-round TKO of Jamie McDonnell in May 2018 to pick up a bantamweight title. Five months later, Inoue dominated a bona fide top-10 bantamweight with a first-round KO of Juan Carlos Payano. His best victory at bantamweight came in his next fight, a second-round TKO of Emmanuel Rodriguez in the World Boxing Super Series to pick up another 118-pound title.
Throughout all three of those bouts, one attribute stood out: Inoue's vision. His ability to pick off opponents' shots while staying in position to take advantage of openings is what makes him dangerous.
"Inoue has great 'eyes' with a steely calm enabling him to see where the openings and proper placement for specific punches are," famed trainer Teddy Atlas told ESPN. "His supreme confidence and belief of himself with these attributes and his sense of timing and foot placement that gives him solid balance for delivery of his punches makes him extremely effective offensively.
"Add in his habits of good cover with his hands, and just overall solid fundamentals and he's a 'Monster' to deal with."
Naoya Inoue defeats Paul Butler by technical knockout to become the undisputed bantamweight champion.
Inoue didn't face much adversity during his first three fights at 118 pounds, all of which didn't make it to the opening bell for Round 3, but that changed in a big way in his fourth fight.
In the World Boxing Super Series final, Inoue stepped up in competition in a bout with future Hall of Famer Nonito Donaire in November 2019. The brutal battle, ESPN's fight of the year displayed just the sort of character Inoue possesses.
The then-26-year-old suffered a broken nose along with a broken orbital bone in the first three rounds, but never wavered. Inoue continued to press forward against an imposing puncher and broke through in Round 11 when he folded Donaire with a body shot.
Donaire seemed on the verge of being stopped, but he somehow heard the final bell as Inoue punished him over the final two rounds despite two injuries that affected his vision and ability to breathe.
"He was tough," Donaire said. "I didn't expect that from him. He can crack, too. ... He's got some speed. He's got the overall [package]," the "Filipino Flash" said on the DAZN Boxing Show ahead of his rematch with Inoue in June 2022. In that second bout, Inoue left no doubt with an absolute destruction, a second-round TKO of Donaire, to add a third bantamweight title.
He sent Donaire to the canvas with a chopping right hand in Round 1 -- Donaire said that's the hardest he's ever been hit -- and then Inoue dropped Donaire again in Round 2 ahead of the TKO victory.
"He's a mean son of a b----," Mike Tyson said on his "Hotboxin'" podcast last year. " ... He don't look like much. He's better than Manny Pacquiao. ... He's vicious. ... He's a f---ing monster."
Hall of Fame promoter Lou DiBella, who oversaw HBO's boxing programming from 1989 to 2000, echoed Tyson's sentiments.
"He's a complete fighter and this differentiates him from a lot of the other Japanese fighters historically: he's a badass," DiBella told ESPN. "He's got a badass killer mentality."
'He's the best fighter ever from Japan'
Naoya Inoue puts on a masterful performance as he scores three knockdowns through three rounds of competition in handily defeating Michael Dasmarinas.
Inoue isn't yet a star stateside. After all, he's only fought in the U.S. three times, with the rest of his bouts taking place in Japan, where he's an icon. Most of his recent fights have been streamed on ESPN+, but in the morning on the east coast, and in the early-morning hours on the west coast.
That lack of visibility has made it difficult for Inoue to break through in the U.S., but there's no questioning what he means in Japan.
He fights before sold-out crowds in his homeland and generates sizable gates. Inoue routinely attracts millions and millions of viewers. And in the boxing-crazed nation, he stands above the rest.
"Japanese boxing has a storied century-old history ... he's the best fighter ever from Japan," said DiBella. "He has no weaknesses. Fulton is a really terrific talent, he's not without a chance here. But 'The Monster' doesn't do anything wrong. You have to fight him so perfectly to have a chance. He's just an extraordinary fighter to watch."
'Very good boxing IQ'
Far more than just a badass boxer with knockout power in both hands and special athletic gifts, Inoue also possesses the ring smarts to accompany those physical traits.
Former junior welterweight contender Dmitry Salita witnessed Inoue's boxing mind firsthand. Salita promoted Antonio Nieves, Inoue's opponent for his U.S. debut, and watched as Inoue toyed with the American on an HBO triple header in September 2017.
"He has very good boxing IQ," Salita told ESPN. " ... With Inoue, similar to [Terence] Crawford, he's a very good judge of distance and knows how to change speed. He knows how to vary his power shots. He sometimes throws punches just to relax his opponents and make them feel safe.
"He has several different levels of power. He has light shots, medium shots and he relaxes his opponent so they think he's a good puncher but not a big, big puncher. He's able to deliver his punches and change his various speeds during the fight and during the round. He's extremely explosive and he's really able to go from 0 to 60 in a matter of milliseconds."
Salita is also impressed with Inoue's disciplined approach.
"He seems like he's been built mentally, physically and spiritually to be a special fighter since a young age. When the going gets tough and you get tired, he's able to pull on that education that he's had since he's a little kid."
Simply put, to quote DiBella, "If you don't love this kid, you don't know anything about boxing."