prior to wrestling professionally, kobayashi had a variety of athletic backgrounds, practicing baseball and kickboxing; the latter came to be a strong foundation in his wrestling style often referred to as shoot style. kobayashi made his professional wrestling debut on may 24, 2000 against future ally and rival, naomichi marufuji, in all japan pro wrestling (ajpw) to compete in their junior heavyweight division. he did not remain in the company long as mitsuharu misawa, acting president of ajpw, decided to secede from the company following a dispute with other forces, thus leading to misawa forming his own company, pro wrestling noah.