In 1998, Kosaka signed with the Ultimate Fighting Championship and debuted at the event UFC 18, facing Kimo Leopoldo. The much heavier Kimo controlled the first minutes of the fight and managed to lock a kneebar, but the Japanese shooter showed his field of expertise by reversing the hold and attempting his own ankle hold, always keeping a strong active defense. At one point, Leopoldo gained the mount, only for Kosaka to reverse the position into a heel hook attempt. As the match progressed, TK started to control an increasingly tired Kimo, landing precise punches and leg kicks until the end of the round. Came the overtime, TK further dominated Leopoldo, stopping his offense and performing ground and pound until the end, capturing the win by unanimous decision.