Takahashi debuted in mixed martial arts in Pancrase against legally blind yet decorated amateur wrestler George Weingeroff. The Japanese capitalized on his opponent's inability to see his strikes, and knocked him out with a roundhouse kick. However, it was in his second professional fight in which he gained popularity, becoming known for his incredible toughness and desire to win. Fighting Ken Shamrock, Takahashi suffered a broken jaw from a palm strike and knee and ankle damage from a heel hook, but refused to give up. Takahashi was limping badly for the rest of the bout but continued to fight. Eventually, he got caught in a very tight heel hook and could not escape and was forced to tap. Takahashi had to be carried out of the ring after the fight. In another memorable showing of heart, Takahashi got caught in a heel hook against Bas Rutten which inevitably broke his shin bone in half. Unwilling to stop the match, Takahashi stood up and continued to fight until, upon kicking Rutten with his broken leg, his shin bone completely snapped in half. His shin didn't bend thanks to the kneeboot he was wearing, but the injury was too much and he finally accepted the match was stopped. It would be a year and a half until Takahashi fought again.
In 1997, Takahashi had his debut for Ultimate Fighting Championship at the UFC 12 event, where he was pitted against Brazilian jiu-jitsu specialist Wallid Ismail. The fight would become infamous for its irregularities and disregard for the rules, as Takahashi seemed to be uninformed of the event's ruleset while Ismail intentionally ignored it. Wallid tried to take Kazuo down several times, only for the Japanese to keep balance every time by illegally grabbing the fence despite the referee's continuous warnings. Minutes into the match, Yoshiki knocked down Wallid with a right hook that seemed to end the fight, but the Japanese wrestler stood waiting instead of following up, believing his opponent would receive a 10 count like in Pancrase. Takahashi was better informed about the rule allowing low blows, however, and he demonstrated it by shockingly sliding his hand on Ismail's trunks, tearing away his protective cup and hitting multiple knee strikes to the groin. After some more strike exchanges, the Japanese took the Brazilian down and landed headbutts and hammerfists through his guard until the end of the round. As there was no finish, they went into extra round. New miscommunications would arise at the overtime when Ismail eye-gouged Kazuo, as the Japanese wrestler asked for time to check it out, which was refused due to referee Big John McCarthy not noticing the foul. Takahashi also kicked Ismail on the ground, an illegal attack under the UFC ruleset due to his wrestling shoes. Once cleared up from the confusion, Takahashi dominated the rest of the overtime with effective punches to take the unanimous decision win. He advanced round, but had to pull out due to a hand injury, being replaced by alternate Nick Sanzo.
