Sumo Grand Champion Harumafuji is suspected of assaulting and injuring fellow wrestler Takanoiwa last month by hitting him with a beer bottle, according to a media report.
Harumafuji was drinking with other Mongolian wrestlers on October 26 after they wrapped up a regional tour to Tottori, according to the report.
It happened at a party of sumo wrestlers from Mongolia.
The victim, Takanoiwa, 27, announced Monday that he would not be taking part in the 15-day Grand Sumo Tournament, which began this weekend in western Japan, according to local public broadcaster NHK.
"As for Takanoiwa's injuries, I apologize deeply for causing trouble for stable master Takanohana, people affiliated with Takanohana stable, the Sumo Association and my stable master", he told reporters. There has been no confirmation that the head injury and allegation are linked, but officials are investigating.
The incident was reported to police but it was unclear if the champion would be charged.
Weighing in at 300 pounds, Harumafuji is considered a lightweight sumo wrestler, and is lauded for his technique in the ring.
Wrestlers are expected to not show emotion after a victory and a rigid hierarchy exists.
That case exposed a culture of bullying and hazing within the ancient sport's strict hierarchy.
A sumo wrestling grand champion is under fire after allegedly hitting a fellow wrestler over the head with a beer bottle.
Three years later in 2010, the sport was rocked by more controversy when Mongolian grand champion Asashoryu retired after being accused of breaking a man's nose in a drunken brawl outside a Tokyo nightclub, AFP reports.