Lesnar began his MMA career in 2007 with a brutal TKO victory over Min-Soo Kim, who submitted due to the onslaught of punches from the Beast Incarnate in the first round in Los Angeles.
The former NCAA heavyweight champion then signed with the UFC later that year and had his debut scheduled against Frank Mir on February 2 2008 at UFC 81.
But despite being a physical specimen and with an incredible 106-5 amateur wrestling record under his belt, Lesnar was nervous about stepping into the Octagon and wanted to get a feel for it before the big bout in Las Vegas.
“It’s a different game, you know, entering the Octagon," he said on The Pat McAfee Show.
“And I’m telling you, you’ve got to be – like, you’ve got to be half bats*** crazy.
“Like when you go through the door, like, I built or ordered an Octagon so I could, didn’t have at first, like, jitters of getting in an Octagon.
“I’m like, ‘Dana, I want an Octagon' and like, ah boom, [the UFC] shipped an Octagon. So I can fight in it and practice in it, you know?
“But then you’re in front of 20,000 people and they shut the door and it’s like, 'Ahhh'.”
Lesnar was submitted by a kneebar from Mir in the opening round and it looked as though his UFC career was already done and dusted.
However, he convinced the promotion to let him fight again and he made amends in his second fight, beating Heath Herring in his hometown of Minneapolis.
Then, three fights in he became UFC heavyweight champion by defeating Randy Couture via TKO at UFC 91, before levelling the score with Mir when he defended his crown in July 2009.
Lesnar added: “I was so mad at myself. It just built another thing that the people ram down your throat and say you can’t do something.
“Love those people. Love them. I get goosebumps for that. I had a one-fight deal, and I was like, 'Please give me another shot at this'.
“Boom, then it took off. I fought Heath Herring and then I got Randy Couture and won the belt.”
Lesnar, who was struggling with diverticulitis during his UFC run, lost the title to Cain Velasquez in October 2010 at UFC 121.
After losing to Alistair Overeem, Lesnar retired from MMA and made a return to WWE.
He did make a winning comeback in July 2016 but his unanimous decision victory over Mark Hunt became a no contest after Lesnar tested positive for banned substance clomiphene.
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