Caol Uno
Before entering the UFC, Thomas held a 12–1 MMA record including wins over future UFC lightweight champion Jens Pulver and Dokonjonosuke Mishima, with his only loss coming to future UFC lightweight title challenger Caol Uno. Thomas made his UFC debut at UFC 32, in a fight against future UFC Lightweight and Welterweight champion B.J. Penn, where he lost by TKO (strikes) in the first round. Thomas returned at UFC 33, defeating Fabiano Iha by unanimous decision.