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.