Billed simply as "Jean Claude" in the promotion, he made his debut against Peter Aerts at the K-1 Grand Prix '96 Opening Battle in Yokohama on March 10, 1996.

[6] After defeating Achille Roger on May 31, 1997 in San Jose, California, Leuyer returned to Japan to fight for the WKBA World Super Heavyweight Championship against Ray Sefo at K-1 Dream '97 on July 20, 1997.

On May 24, 1998, Leuyer fought Maurice Smith for the Draka World Super Heavyweight Championship in Los Angeles, California, but the bout was ruled a draw.

[9] He made a defence of his ISKA World Super Heavyweight Freestyle Championship against Andrei Dudko on May 15, 2000 in San Jose, California, stopping the challenger with a low kick in the fifth round.

[12] Leuyer won back his ISKA World Super Heavyweight Freestyle title on September 29, 2001 in San Jose when he defeated Jeff Ford by TKO in round eight.

His last foray into K-1 came the following year on May 3 at the K-1 World Grand Prix 2002 Preliminary USA in Las Vegas where he was defeated via unanimous decision by Dewey Cooper in the quarter-finals.