 In 2006 , Lesnar pursued a career in mixed martial arts . He signed with Hero 's and won his first fight , against Min @-@ Soo Kim , in June 2007 . He then signed with the Ultimate Fighting Championship ( UFC ) the following October . Lesnar lost in his UFC debut against Frank Mir and then won his second fight against Heath Herring . In November 2008 , Lesnar defeated Randy Couture to become the UFC Heavyweight Champion . Shortly after a successful title defense in a rematch with Mir , Lesnar was sidelined due to diverticulitis . He would return at UFC 116 to defeat Interim UFC Heavyweight Champion Shane Carwin and unify the heavyweight championships , becoming the Undisputed Heavyweight Champion . Lesnar then lost the championship to Cain Velasquez at UFC 121 . In 2011 , he was once again sidelined due to diverticulitis and underwent surgery . Lesnar returned at UFC 141 in December , losing to Alistair <unk> and promptly retiring from MMA . Lesnar was a box office sensation in UFC . He took part in a few of the best selling pay @-@ per @-@ views in UFC history , including UFC 100 and UFC 200 . 
