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Four Weight Boxing World Champ Retires At 29 Due To Injury

Originally posted on KO on SI | By Grant Young | Last updated Jun 4, 2025 12:31 PM ET

While they aren't the biggest stars in America or Europe, true boxing fans know that some of the world's greatest sweet science practitioners hail from Japan. There's no better example of this than Naoya Inoue, who is considered one of the pound-for-pound greats in the sport right now. Another well-known Japanese fighter is Junto Nakatani, who Inoue could be fighting later this year if all goes according to plan.

However, one of Japan's most intriguing fighters won't be making another walk into the ring.

This is 29-year-old Kosei Tanaka, who announced his retirement from the sport on June 4 because of an injury. Tanaka made this announcement with an Instagram post that read (translated to English):

"I'm retiring as a professional boxer... The reason is all due to repeated eye injuries. THe last defense match was held on 10/14. The condition of both eyes was bad before the match, and right after the match, I decided to have surgery on both eyes and went into the match.

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A post shared by 田中恒成 (@koseitanaka_5)

Tanaka later added in the post, "Although my vision has recovered, my vision in my right eye is still significantly distorted and I can't focus with both eyes. Due to the effects of hernia surgeries on my neck and many, many surgeries on my eyes alone in the four years from 2021, my eyes became brittle and I was unable to spar, let alone compete. I decided to retire because there was no way for me to get into the ring."

Tanaka will retire as a former four-weight division champion, as he held the WBO mini-flyweight belt from 2015 to 2016, the WBO junior-flyweight belt from 2016 to 2017, the WBO flyweight belt from 2018 to 2020, and the WBO junior-bantamweight belt in 2024. He also became the fastest Japanese fighter ever to become a world champion upon winning his first belt in 2014, just five fights into his professional career.

If Tanaka is truly hanging up the gloves, he'll finish his career with a 20-2 record with 11 KOs. His final fight will be a loss to Phumelela Cafu in October 2024.

This article first appeared on KO on SI and was syndicated with permission.

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