Junto Nakatani tightened his grip on the bantamweight division after Ryosuke Nishida retired with an eye injury before the start of Round 7 of their world title unification bout on Sunday.
Nakatani added Nishida's IBF title to his WBC belt to remain No. 1 in the division after the accuracy of his punches closed shut Nishida's right eye, prompting an inspection from the ringside doctor and the decision to retire him following the sixth round at the Ariake Colosseum in Tokyo, Japan.
Naoya Inoue, who is being linked with a huge fight versus Nakatani next year, watched from ringside as Nakatani took control in rounds five and six against his fellow Japanese fighter in a fourth defense of his WBC title.
Three-weight world champion Nakatani (31-0, 24 KOs), 27, of Kanagawa, Japan, will now contemplate stepping up to junior featherweight or going for another world bantamweight title unification fight.
The other world champions at 118 pounds are Antonio Vargas (WBA) and Yoshiki Takei (WBO). A megafight versus Inoue is being talked about for next year, so Nakatani is expected to have a fight in the interim.
A clash against Inoue (30-0, 27 KOs), ESPN's No. 2 ranked pound-for-pound boxer and undisputed world junior featherweight champion, would be the biggest fight in Japanese boxing history.
Nakatani is ranked at No. 9 in ESPN's latest pound-for-pound list and this win edges him closer to Inoue. Southpaw Nishida (10-1, 2 KOs), 28, contributed to an absorbing fight that saw both sustain a furious pace for six rounds.
Five-foot-eight Nakatani, who trains in Los Angeles with coach Rudy Hernandez, has now won all five of his fights at bantamweight by stoppage and he began as if he wanted to end it early, with an all-out attack in Round 1.
Both let their hands go from the first bell but Nakatani had the better of the early exchanges, including a big overhand left and a sequence of uppercuts in Round 2.
Nishida was better in the third and fourth rounds as he targeted the body, but his right eye began to swell in Round 5 and by the end of Round 6 it was closed shut. Nakatani mercilessly targeted Nishida's swollen eye with stinging left hands as he began to land shots with more regularity.
Defeat looked inevitable for Nishida considering he could only see out of one eye and after an assessment of the eye injury by the ringside doctor, it was a sensible decision to pull him out of the fight after the sixth round.
On the same bill, Japanese bantamweight Riku Masuda (8-1, 8 KOs) delivered a concussive southpaw one-two that knocked out Michell Banquez, of Venezuela, inside a round.