Former ozeki Mitakeumi upset Onosato on a day of upsets on Sunday, July 14, the opening day of the Nagoya Basho.
No 2 maegashira Mitakeumi quickly got a left hand inside on his opponent at Aichi Prefectural Gymnasium and used his right hand to shove out the stunned new sekiwake.
Onosato comes into the 15-day July tournament as the favorite after the pride of Ishikawa Prefecture won the previous tournament for his first Emperor's Cup.
It was the first-ever meeting between the two wrestlers and veteran Mitakeumi took advantage of his experience to overpower the 24-year-old up-and-coming grappler who couldn't mount any counterattack.
In the day's final bout, grand champion Terunofuji got an arm lock on Hiradoumi immediately after the face-off and forced the komusubi out over the edge to kick off his Nagoya campaign with a win.
Terunofuji captured the New Year tournament title for his ninth Emperor's Cup but then was unable to fully complete the next two tournaments due to injury.
He is bidding for a 10th title here which would allow him to go down in the record books as a great yokozuna. He looked in fine form on Sunday but it remains to be seen if he can complete a full tournament.
Atamifuji Manhandles Hoshoryu in Nagoya Basho Showdown
In other major Nagoya Grand Sumo Tournament bouts, top maegashira Atamifuji used his 40-kilogram weight advantage when he got a left-hand grip on the belt of ozeki Hoshoryu before forcing the ozeki out to an opening-day loss.
Also Sunday, in another big upset, komusubi Daieisho used an arm thrust to the neck to send Kotozakura out over the straw ridge in a matter of seconds.
Ozeki Kotozakura came into the tournament as a title favorite. But he will have to show more fighting spirit than he did on Day 1 to have a shot at the Emperor's Cup.
Top maegashira Meisei used a frontal push-out after a strong initial charge to force out relegation-threatened ozeki Takakeisho, who showed little resistance.
"I wanted to hit him hard at the initial charge and my body was moving well," said Meisei. "I've always competed well in Nagoya so I want to continue what I have been doing in the past."
Takakeisho bowed out of the May tournament after an opening-day loss to Hiradoumi and didn't look like a man determined to hang on to his ozeki rank on the first day.
Wakamotoharu Begins Nagoya Basho with a Win
No 2 maegashira Wakamotoharu took a page out of his opponent's playbook when he used several powerful arm thrusts to send sekiwake Abi over the straw ridge.
In a showdown of former ozeki wrestlers looking to overcome injury concerns, Kirishima got a right-hand grip on the belt of Takayasu and shoved the No 3 maegashira out to start off with a win.
Mongolian sekiwake Kirishima could return to the second-highest rank of ozeki if he is able to win 10 bouts or more in the Nagoya Basho.
Both grapplers missed parts of the previous tournament with injuries and are looking to get back on track.