Nine-time grand champion Mongolian wrestler Harumafuji, 33, has apologised for "causing trouble".
The alleged victim, Takanoiwa, was hospitalised for several days, the Japan Sumo Association has said.
Sumo association officials told AFP news agency that exactly what happened remains unconfirmed.
Takanoiwa, who is also Mongolian, is reported to have suffered a fractured skull. The 27-year-old is part of a so-called 'stable' led by Takanohana, a former grand champion who reported the incident to police, according to Kyodo news agency.
Harumafuji and his stable master, Isegahama, were questioned by association executives on Tuesday.
The grand champion apologised publicly but did not confirm the circumstances of the incident.
"As for Takanoiwa's injuries, I apologise deeply for causing trouble for stable master Takanohana, people affiliated with Takanohana stable, the Sumo Association and my stable master," he told reporters.
Weighing in at 137kg (300lb), Harumafuji is considered a relatively small sumo wrestler, and is lauded for his technique in the ring.
Sumo's origins lie in Shinto rites performed in temples, and Japanese fans expect wrestlers competing in the ancient sport to live up to strict standards of good behaviour.
Wrestlers are expected to not show emotion after a victory and a rigid hierarchy exists.
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