Grand champion Hakuho was sent to a shock defeat at the hands of Aminishiki, while sumo's six ozeki, featuring newly promoted Mongolian Kakuryu, got off to flying starts at the 15-day meet on Sunday.
Kakuryu, who narrowly missed his first career victory after falling to Hakuho in a playoff at the spring basho in March, dispensed with would-be challenger Gagamaru in dominant fashion by dumping his larger opponent with a sure-fire underarm throw at Ryogoku Kokugikan.
But Hakuho, who is seeking his 23rd career championship, was knocked off balance after a slow jump at the charge before the komusubi abruptly upended the yokozuna.
''I got a great charge at the face-off,'' said Aminishiki, who needed just four seconds to send the yokozuna over the edge. ''My reaction time was very quick. I just was focused on trying to do my style of sumo.''
Hakuho, who is aiming to surpass former yokozuna Takanohana to sit alone in fifth on the all-time title winners' list, started a meet with a defeat for the first time since the 2008 Kyushu tournament.
It is the first time ever that six ozeki are appearing at the same meet, which more than anything else is an indication of the second-highest rank's inability to move up the ladder to sumo's highest rank.
Bulgarian ozeki Kotooshu laid the pressure on thick with a salvo of slaps as he charged out Myogiryu to get started in the winner's circle.
Meanwhile Kotoshogiku, who is nursing a left wrist injury, wrapped his left hand around Takayasu for a solid grapple on the upper body before escorting his opponent over the straw bales.
Kisenosato bulldozed Aran in a lopsided affair, unleashing a series of quick thrusts before plowing out his Russian opponent.
The Naruto stable wrestler is said to have the best chance of becoming the first Japanese in more than six years to capture a title.
Goliath Baruto used a lift out maneuver against Toyonoshima, tossing the diminutive wrestler like a rag doll when his sekiwake opponent attempted an ill-advised frontal grab.
Mongolian Harumafuji slapped down Homasho after a short exchange of shoves, making it 6-for-6 for the ozeki rank.
An official summer meet is being held for the first time in two years after last year's basho was made free to the public as a technical ''test'' tournament in the wake of a match-fixing scandal.