Hakuho was all business in the day's finale, the Mongolian grand champion yanking down Yoshikaze by the back of the neck and squashing second-ranked maegashira to the dirt after he had tried to counter with a thigh grab.
Hakuho, who is gunning for his sixth straight Emperor's Cup and 18th overall title, is tied at 5-0 with former sekiwake Wakanosato and 16th-ranked maegashira Tochinonada. Harumafuji dropped out of the pack after a surprise defeat.
Harumafuji's unbeaten start came to an end when he was outmuscled by komusubi Tochiozan (2-3) in the day's main upset at Ryogoku Kokugikan.
Harumafuji pulled out of the Kyushu meet in November with a right ankle injury and must post at least eight wins here to preserve his ozeki status.
''I wanted to get forward and attack and was able to do that,'' said Tochiozan. ''Hopefully I can continue wrestling like this. I want to try and match the expectations people have of me.''
Kotooshu never looked in any danger of following suit and biting the dust in the next bout, the burly Bulgarian getting inside Tamawashi (2-3) and spinning the third-ranked maegashira around and down to the sandy surface.
All ozeki wrestlers finished the day at 4-1 as Kaio and Estonian giant Baruto both won their respective bouts.
Baruto secured European bragging rights by huffing and puffing his way past winless Georgian No. 2 maegashira Tochinoshin before Kaio efficiently disposed of top-ranked Toyonoshima with a trademark ''oshidashi'' shove.
Toyonoshima, who went 14-1 before falling to Hakuho in a playoff at the Kyushu basho in November, came into the New Year meet struggling with back pain and has only won one of his first five bouts.
At sumo's third-highest rank of sekiwake, Kisenosato, the man who snapped at 63 Hakuho's winning streak in Kyushu, remained one win off the pace after bundling over top-ranked Aminishiki (1-4).
Kisenosato's win restored a little pride for his rank after Kotoshogiku was condemned to his first loss at the hands of Mongolian komusubi Kakuryu (1-4) in the preceding bout.