TOKYO (Kyodo) -- Japanese boxing superstar Naoya Inoue comfortably retained the undisputed super bantamweight world championship Friday with a fourth-round knockout of South Korea's Kim Ye Joon.

The 31-year-old power puncher, known by the nickname "Monster," looked in control from the outset at Tokyo's Ariake Arena against Kim, who stepped in as a late replacement for injured Australian challenger Sam Goodman.

Inoue inflicted early damage when he caught Kim below the eye with a right jab in the second, necessitating treatment for the South Korean between rounds.

Kim looked to answer in the third and had success with a solid combination around a minute into the round before Inoue reasserted himself.

The champion turned up the intensity from the start of the fourth and had Kim backpedaling before sending him down for the count with a devastating straight right 2 minutes, 25 seconds into the round.

Inoue improved to 29-0, including 26 wins by knockout, with his third defense of the undisputed crown since unifying all four belts with his victory over Marlon Tapales in December 2023. The 32-year-old Kim dropped to 21-3-2 with 13 KOs.

"I took a few more punches than usual, but that was due to my lack of preparation after the sudden change of opponent," Inoue said.

"I was trying to figure him out in the ring, and my overall performance was very good."

Inoue had originally been scheduled for a Dec. 24 fight with Goodman, the top-ranked IBF and WBO contender, before the Australian suffered a cut above his left eye, forcing a postponement.

Goodman pulled out completely on Jan. 11 after aggravating the injury, with Kim, who was ranked 11th by the WBO, stepping up to challenge Inoue in his place.

"I'm tired, not because of the bout but with what has happened over the last two months. It's not been tough physically, but mentally," Inoue said.

Fight promoter Bob Arum stepped into the ring after the match and announced that Inoue plans to fight this spring in Las Vegas. The boxer also revealed plans for a bout in Saudi Arabia.

"I'm aiming to fight overseas this year myself...so please stay tuned," Inoue said.

"I don't think I'm moving up my division soon. I believe I still need to fight in the super bantamweight."