During the mid-nineties when the UFC was becoming a well-known name, many fighters from Japan's 'Pancrase' organization fought for the UFC. Fighters such as Frank Shamrock (first UFC Light Heavyweight Champion), Ken Shamrock (UFC Superfight Champion, UFC 3 Tournament Finalist, UFC 1 & UFC Ultimate Ultimate Tournament Semi Finalist & UFC Hall of Famer), Bas Rutten (UFC Heavyweight Champion & UFC Hall of Famer) & Guy Mezger (UFC 13 Lightweight Tournament winner). Guy Mezger is looked down upon when compared to his Pancrase buddies & it's due to the competition he faced whilst in the UFC. The toughest opponent Mezger faced was future UFC Hall of Famer Tito Ortiz and the two exchange wins with each other over the course of 2 years. After Mezger was TKO'ed by Ortiz at UFC 19 he spent most of his finals years in pride going 4-6 in the promotion, while he lost more than he won in Pride he did face much tougher competition as he went toe to toe with Wanderlei Silva (knockout loss), Kazushi Sakuraba (corner stoppage loss), Chuck Liddell (knockout loss), Ricardo Arona (split decision loss), Antonio "Lil Nog" Nogueira (split decision loss) & Egan Inoue (knockout win).

In the lead-up to a scheduled rubber match with Tito Ortiz at UFC 50 in October of 2004, Mezger was admitted to a hospital after suffering "stroke-like" symptoms. He would decide to end his fighting career after this health scare, his final fight being a win over Daniel Bergman in a European Vale Tudo competition in December of 2003.