Komusubi Kotonowaka maneuvered Hoshoryu to the edge and shoved him out after getting a grip on his belt to end a 10-bout losing streak against the Mongolian grappler.
While it was a huge upset win for Kotonowaka, who improved to 6-4, it was a painful loss for Hoshoryu, who dropped to 8-2 in the 15-day Nagoya Grand Sumo Tournament at Aichi Prefectural Gymnasium.
Hoshoryu can still get the 12 wins he needs to earn promotion to ozeki and win the tournament. But Tuesday's loss was a setback for a wrestler who has looked dominant and had history on his side.
In another major bout, top maegashira Nishikigi slipped his left arm under the arm of Meisei and deployed a superb frontal crush-out to pick up his ninth win against a lone loss. Third-ranked maegashira Meisei dropped to 4-6.
No 9 maegashira Hokutofuji maintained his share of the lead at 9-1 when he thrust down seventh-ranked maegashira Tamawashi, who lost his footing and dropped to 6-4.
Sekiwake Daieisho (8-2) deployed a thrust-down technique at the edge to defeat No 5 maegashira Hiradoumi (3-7) and wrap up a winning record.
Daieisho needs three more wins in his remaining five bouts to get to 33 over his last three tournaments, a number that would likely earn him promotion to sumo's second-highest rank of ozeki.
"My face-off was good today," said Daieisho, who will face ozeki Kirishima on Day 11. "I just have to fight my own brand of sumo and not try to force things."
Onosho Conquers Wakamotoharu
No 5 maegashira Onosho dealt a serious blow to Wakamotoharu's ozeki promotion bid when he came in low after the face-off and drove the sekiwake out over the straw ridge.
Onosho improved to 5-5 while Wakamotoharu fell to 7-3. For Wakamotoharu to get the 12 wins he needs for possible promotion, he would need to win all of his remaining five bouts, including Wednesday's showdown with Hokutofuji.
Ozeki Kirishima dispatched No. 4 maegashira Ura in the day's final bout to improve to four wins, four losses and two absences. Ura fell to 4-6.
Further down the ranks, No 16 maegashira Endo stayed one win off the pace along with Hoshoryu and Daieisho when he shoved out No 12 Chiyoshoma (5-5) to improve to 8-2.