Critics called the WWE superstar a sideshow attraction but, while his UFC 81 fight had a shock ending, he proved them wrong and set up a fierce rivalry with Frank Mir.
Lesnar joined the UFC's roster in October 2007 in a storm of publicity. Famous after his years topping the bill in WWE, many anticipated Lesnar struggling badly as he swapped staged fighting for the real thing.
His Las Vegas MMA debut came at an event billed as 'Breaking Point' against Mir in 2008. Lesnar had only one previous fight - a victory over Kim Min-soo in Japan - while the man who welcomed him to the Octagon was a former UFC heavyweight champion. Merciless matchmaking.
Mir had a huge experience advantage over Lesnar who had stopped Kim in just 69 seconds - and his UFC debut wouldn't last much longer.
Lesnar came out all guns blazing and made a tremendous start, securing an early takedown then raining down punishment on his foe.
Mir could thank his lucky stars that referee Steve Mazzagatti controversially intervened, adjudging that Lesnar had landed an illegal blow to the back of the head. Lesnar was deducted a point with the fight resuming from a standing position.
Lesnar had lost his advantageous position but dropped Mir again from the restart, then landed a flurry of hammerfists as he eyed a first-round victory.
However, Mir reversed position after absorbing some serious damage, grabbed hold of Lesnar's leg and eventually locked in a nasty kneebar which forced 'The Beast Incarnate' to tap.
Lesnar was on the brink of announcing himself to the MMA world in stunning style until he presented Mir with the smallest of openings.
It was a harsh introduction to the UFC but an essential building block for Lesnar's fledgling MMA career. Even in defeat, he had shown his critics he was no pro-wrestling faker.
When he faced Mir in a bad-tempered 2009 rematch, Lesnar was the UFC champ - and the outcome was very different.
Topics: UFC News, UFC 81, Fight News, UFC, MMA, Brock Lesnar
Nasir Jabbar