Fighting Network RINGS 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. He won the tournament by knocking out Hiroyuki Yoshioka, submitting Susumu Yamasaki with a
heel hook, and finally outclassing
Brazilian jiu-jitsu champion
Egan Inoue at the finals with a mix of judo throws and positional reversals. After his return to RINGS, Kosaka was put in opening shoot matches, defeating names like
Maurice Smith and
Willie Peeters. In September 26, 1997, however, he was thrown to a greater challenge when he faced former
Pancrase star
Frank Shamrock. Tsuyoshi controlled the match, throwing and taking down the lighter Shamrock and threatening him with submissions, but couldn't lock any of them, while Shamrock was successful at defending from
half guard and counterattacking with his own hold attempts. After a failed takedown, the judoka was caught in a
guillotine choke by Frank and had to spend a rope escape. 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. In April 1999, after a series of matches in UFC, Kohsaka fought Dutch kickboxer
Gilbert Yvel in RINGS. The match saw a worn Kosaka getting ahead in points by multiple fouls from Yvel, but he failed at submitting him while Yvel landed several knockdowns, until one of them caused the fight to be stopped. They rematched in August, when an improved Kohsaka immediately threw Yvel down, hit him with numerous body punches and submitted him with a
toehold for a rope escape; however, the bout was interrupted for Kohsaka falling out the ring in another hip throw, getting injured. As TK had scored a point before the bout went to no contest, the match was counted as a victory for him. Finally, Yvel and Kohsaka had a rubber match during the King of Kings Tournament, but it was short and uneventful, as TK was stopped again by a bleeding cut. In August 2000, TK faced the King of Kings semi-finalist
Antônio Rodrigo Nogueira. Started the fight, Kohsaka blocked a takedown and transitioned it into an
uchi mata, but Nogueira pulled him to the ground and
mount him. The judoka escaped from under him, only for Nogueira to regain position, but this time it was Kohsaka who reversed him into a
leglock attempt and pressed into his guard when it failed. 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. Nogueira mounted him again and threw punches, but Kohsaka performed again his signature escape into leglock and keep attacking his guard. A pair of sweeps later, Kohsaka controlled position until the end of the time. The match was ruled a draw. During the next King of Kings tournament, Kosaka advanced over
Mikhail Ilyukhin by knockout and Mikhail's teammate
Fedor Emelianenko by doctor stoppage. The latter was again a controversial technical victory, as the cut was caused by a missed looping punch where Kosaka's elbow struck Emelianenko's head, reopening a cut sustained in his previous fight against
Ricardo Arona. 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. At the finals, TK faced
Randy Couture in a back and forth match. 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. The match ended with a trade of strikes in which Couture got the best part, being granted the split decision and eliminating the Japanese. In June 2001, Kosaka fought luta livre specialist
Renato Sobral, who had defeated Kosaka's countrymen Kiyoshi Tamura and Hiromitsu Kanehara. Sobral controlled the stand-up segments via wrestling and knee strikes, while Kosaka worked from the bottom and pursued submissions. 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. Victory by majority decision was given to Sobral. After a quick KO victory over RINGS Russia exponent Koba Tkeshelashvili in August, Kosaka fought Koba's teammate
Bazigit Atajev in December 2001 in what would be his last fight in RINGS. The judoka dominated the earlier segments of the match, taking down Atajev and exerting both ground and pound and armlocks attempts, but the Russian changed the tide of the match thanks to his superior stand-up, which drew blood from TK. At the end, Atajev won a majority decision.
Ultimate Fighting Championship In 1998, Kosaka signed with the
Ultimate Fighting Championship and debuted at the event
UFC 16, 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. Later that year TK fought
Pete Williams at
UFC Brazil. Williams went aggressively against Kosaka using his vaunted striking, but Kosaka repeatedly threw him down with
uchi mata and transitioned them into armlock attempts, using his defensive guard whenever Williams managed to get back the upper hand. At the end, Tsuyoshi won again by unanimous decision, which gained him a spot in a tournament for the title that was vacated by
Randy Couture. At 1999's
UFC 18, Kosaka returned to UFC fighting the legendary
Bas Rutten in the first round of the "Road to the Heavyweight Title", which was a four-man tournament that would crown the next
UFC Heavyweight Champion. 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. The fight was a source of heated controversy because referee
John McCarthy seemed to unfairly stand the fight up when Kohsaka was mounted on Rutten and actively landing clean effective punches. This ordeal would be noted by Jake Rossen of
Black Belt magazine, who wrote: "Overall, the event demonstrated an impatience for lengthy ground battles. Fighters were stood up after just a few minutes of grappling, in an apparent effort to maintain action in the matches. [...] This puts strikers, once the underdog of MMA competition, in a prime position to climb to the top." In the same year, TK faced
Tim Lajcik at
UFC 21, where he won via
TKO when Lajcik quit between rounds. 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. Despite carrying injuries from his grueling year schedule, Kosaka also took part in the second UFC event in Japanese soil,
UFC 23, where he was pitted against Ruas Vale Tudo exponent
Pedro Rizzo. The two contenders spent a round cautiously, until Rizzo switched to aggressive leg kicks and Kohsaka tried to take him down in return. Through the match, Pedro blocked TK's efforts to get it to the ground and landed more punches and leg kicks. After Kohsaka had absorbed a long punishment, Rizzo landed a right punch for the TKO. Tsuyoshi's last fight in UFC was at
UFC 37, fighting
Ricco Rodriguez. 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.
PRIDE Fighting Championships After RINGS's demise, Kosaka wandered between
Pancrase and the MMA division of
New Japan Pro-Wrestling. He then landed in
PRIDE Fighting Championships, in which his first fight would be a rematch against
Fedor Emelianenko for the latter's technical defeat in RINGS, taking place in PRIDE Bushido 6. The fight was one-sided and brutal, with Fedor landing strikes through Kosaka's guard while the Japanese tried unsuccessfully to counterattack with leglocks from the bottom. At the end, after taking punches during most of the round, a bloody Kosaka's corner threw the towel between rounds. Kosaka returned in
PRIDE 31 against legendary Brazilian grappler and
Brazilian Top Team coach
Mario Sperry. The bout was even shorter, as Kosaka knocked him out with punches after a brief struggle. Knowing the end of his career was near, TK announced his intentions to retire should he lose any of his matches in 2006. The day of his fight with Sperry was also the same day of his son's birth. At
PRIDE Total Elimination Absolute, Kosaka fought
Mark Hunt in what would be his last fight. Due to the weight difference between both fighters, knees and kicks were not allowed on the ground. 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. At the second round, the Japanese again tried unsuccessfully to grapple with Hunt, and after long series of strikes the kickboxer knocked him out with a right punch, finally ending the match.
Post-PRIDE 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. He also would defeat the 50 ib heavier
Ricardo Morais in a MMA match for NJPW. He also fought for
Deep, facing
Antônio Rogério Nogueira,
Antônio Rodrigo's brother. Kosaka scored takedowns and throws, but he found himself on the defensor role, which he successfully played by using his classic reversals. 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. The bout went to decision, with Nogueira coming over. KosaKa would go to
Pancrase, defeating the similarly much heavier
Ron Waterman for the Pancrase Super Heavyweight Championship. He returned to the company for a match against
Mike Kyle, losing by doctor decision in a controversial fashion, as Kyle
eye-gouged Kosaka and actually damaged his
cornea without being penalized.
RIZIN Fighting Federation Rizin Fighting Federation announced Kosaka would debut for the promotion against
James Thompson. The bout took place on December 29, 2015 and Kohsaka won the fight via TKO in the second round after controlling the fight with punches and knees. Kohsaka's next fight will be against Sumo Baruto Kaito December 29, 2016 as part of Rizin's Openweight tournament. Kosaka faced
Mirko Cro Cop on December 31 at
RIZIN World Grand Prix 2017: Final Round. He lost the fight via TKO early in the first round. Kosaka faced the Kyokushin Karate World Champion
Mikio Ueda on April 17, 2022 at
Rizin 35. Kosaka announced that this bout will be his last and he will retire afterwards. He won the fight by a first-round stoppage. ==Fighting style==