Twelve years ago, when Takuma Inoue and Seiya Tsutsumi were promising 16-year-old amateur boxers, Inoue defeated Tsutsumi in the semifinal of a national tournament. With decidedly higher stakes Sunday in the main event of a four-title-fight card at Ariake Arena in Tokyo, Tsutsumi got his revenge and upset Inoue by unanimous decision to claim the WBA bantamweight title.

Inoue, 20-2 (5 KOs), the younger brother of 122-pound champ Naoya “The Monster” Inoue, left the ring immediately after his second defense of the alphabet belt proved unsuccessful. Tsutsumi, 12-0-2 (8 KOs), stuck around to fight back tears during an emotional interview following his first scheduled 12-rounder as a pro.

“I dedicated my whole life to this moment,” Tsutsumi said. “I still cannot believe this is happening. … It feels surreal to me.” While wearing his new bantamweight strap, he noted that, as a teen, he’d vowed never to touch any kind of championship belt until he won one himself.

Tsutsumi, 28, entered the ring to Steppenwolf’s “Born To Be Wild,” and while his approach probably shouldn’t be categorized as “wild,” he was certainly the more offensive-minded fighter throughout. He circled nonstop and seemed to be burning nervous energy in the early rounds against a calm, patient, sharpshooting Inoue, also 28. Inoue backed the challenger off with a tremendous counter right uppercut in the third, and through the first four rounds, the fight seemed roughly even.

Starting in the fifth, however, Tsutsumi took over. Inoue got away from his jab and Tsutsumi began to force the all-action fight he wanted. Tsutsumi dominated round 5 and did damage to the body in the sixth, while Inoue, searching for opportunities to counter, couldn’t find any.

Early in round 10, Inoue got caught taking his eyes off his opponent — seemingly glancing up at the clock even though the round had just begun — and Tsutsumi took advantage and landed a shot that knocked him off-balance, into the ropes. Referee Noguto Ikehara ruled that those ropes had held Inoue up and declared a knockdown, over Inoue’s demonstrative objections. It now appeared likely that Inoue would need a knockout to win — and he got a glimmer of hope when Tsutsumi suffered a cut on his left eyelid just before the bell.

Fighting past round 10 for the first time, Tsutsumi held off a dramatic surge from Inoue in the 11th and clearly outfought him in the 12th. The final 30 seconds told the story of the fight, with Tsutsumi pursuing Inoue along the ropes and the defending titlist simply unable to keep up with him, barely throwing any punches as the clock ticked down.

The 10th round knockdown call was decisive on Koji Tanaka’s 114-113 card, while Michiyaki Someya scored the bout 115-112 and Kunihiro Iwasaki saw it 117-110, all for new beltholder Tsutsumi.