After a year, he joined shoot-style promotion Fighting Network RINGS where he began training in catch wrestling under mentor Akira Maeda.
Kosaka had his first mixed martial arts venture when, thanks to his strong judo background, he was sent to represent RINGS in the gi-clad MMA event Lumax Cup 1995.
Even although Kohsaka later clamped a Kimura lock which appeared to carry potential to finish the match, the time ended without Kosaka completing the hold, thus giving the win to Shamrock for his scored point.
The loss, however, was instrumental in Kohsaka's career, as it made him close friends with Frank Shamrock and Maurice Smith, who became his training partners in the team The Alliance shortly after.
At the next round, Kohsaka stopped another takedown with a leg hook tawara gaeshi and continued pressing, blocking Nogueira's attempts to work submissions from the bottom and striking when possible.
Since the fight was in a tournament format, a winner and loser were required because draws and no-contests were not awarded, and as Emelianenko could not advance due to the injury, Kosaka did.
Kosaka controlled the first round of a match fought mostly in the clinch, throwing down the wrestling champion with a harai makikomi and landing abundant ground and pound, but his activity stopped when he received an accidental headbutt.
After the injury was checked, Couture took Kosaka down, and although the judoka used his "TK scissors" move to counter an unfavourable north/south position, the American still controlled the second round.
Action increased at the second round, where Kohsaka and Sobral exchanged positions and reversals, but the Brazilian kept the dominance and it forced Kosaka to resort to sacrifice techniques like kani basami and flying kneebars.
As the match progressed, TK started to control an increasingly tired Kimo, landing precise punches and leg kicks until the end of the round.
Kohsaka dominated most of the fight with his superiority in wrestling and ground and pound, but ultimately lost by TKO when Rutten recovered in overtime and knocked him out with knees and punches.
[8] This ordeal would be noted by Jake Rossen of Black Belt magazine, who wrote: "Overall, the event demonstrated an impatience for lengthy ground battles.
Kohsaka again utilized a strategy of strong defense and skilled reversals to set up submissions attempts, eventually sweeping Lajcik against the cage wall and performing a lengthy ground and pound sequence which made Tim's corner throw the towel after the round.
The fight featured long, back and forth grappling action, with Rodriguez taking dominant position and Kosaka reversing and blocking him, until Ricco finally secured a mount and threw punches to the face for the TKO victory.
Effectively, TK struggled with the much heavier kickboxer, taking his back and performing an inverted triangle/keylock combination only for Hunt to force his way out via raw strength.
Back to standing, Kosaka traded strikes with Hunt and landed effective hits despite the difference in level, absorbing heavy punches and delivering in return combos against the ropes.
[13] From 2003 to 2005, he worked sporadically in professional wrestling for New Japan Pro-Wrestling, challenging Yoshihiro Takayama for the NWF Heavyweight Championship and feuding with Yuji Nagata.
On the third round, he was outstruck and received punches on the ground, but he retaliated at the final time with a toehold attempt from inverted guard, and ended the bout taking down Nogueira.
[23][24] Along with his grappling, Kohsaka made use of an intelligent striking game to wear down his adversary, and he later gained significant punching power after training with Maurice Smith.
[29] In 2012, Kosaka received an honorary Brazilian jiu-jitsu black belt from Yuki Nakai and was appointed a consultant member for the Japanese BJJ Federation.