Despite the oddsmakers ruling against them, these fighters defied expectations to get their hand raised and become UFC champions.
Event: UFC 69
Opponent: Georges St-Pierre
One of the most iconic underdog victories ever to place inside the Octagon, Matt Serra's first-round demolishing of GSP still belies belief.
After all, unlike most UFC title challengers who have to defeat several top contenders before getting a shot at the belt, Serra's title challenge had instead come as a result of him winning the Ultimate Fighter Season 4.
Serra emerged from the season victorious, but a razor-thin victory over fellow UFC vet Chris Lytle in the show's finale didn't exactly inspire confidence for the New Yorker's chances against GSP.
However, when the pair finally met in the Octagon at UFC 69, Serra demonstrated none of the apprehension you might assume someone was going through during the biggest opportunity of their lives.
Unphased by the gravity of the occasion, Serra took the initiative from the opening bell, clipping the champion with a hard right hand before following up with unrelenting pressure forcing John McCarthy to stop the fight in one of the most unlikely upsets, not just in the history of the UFC, but in sport as a whole.
Event: UFC 269
Opponent: Amanda Nunes
The most recent upset to feature on this list, Juliana Pena's route to world champion status was undoubtedly one of the most unlikely in the history of the UFC.
After all, Pena entered the cage to face off against the GOAT of women's MMA having won just two of her last four fights, whilst Nunes was riding a twelve-fight win streak.
In what has become something of a hallmark of the UFC's women's bantamweight division, the belt would change hands in a particularly violent manner, par for the course in a weight class where every title change has come inside the distance.
Unlike many of Nunes' prior victims, Pena was not deterred by the Brazilians knockout power, and began landing her jab with regularity as the fight wore on.
By the second round, Nunes' legendary power had significantly deteriorated and the Lioness appeared exhausted. Pena continued to turn up the pace of the action, forcing the champion against the cage and completing a takedown. Then, after securing a dominant position, Pena sunk in a rear-naked choke, and forced Nunes to submit, becoming just the fifth ever champion of the UFC's longest-running women's division.
Event: UFC 173
Opponent: Renan Barao
Based on their career trajectories in the years since, you would be forgiven for not realizing that sure-fire future UFC Hall of Famer TJ Dillashaw would ever have opened as a betting underdog against Renan Barao.
After all, with nearly a decade having passed since the fateful fight, Dillashaw remains among the elite at 135lbs while Barao is currently riding a five-fight losing streak and has been released from the promotion he was once champion in.
But 2014 was an entirely different world.
With a record of 32-1, Renan Barao was one of the most frightening prospects ever seen in the lighter weight classes. In the absence of an injured Dominic Cruz, Barao had established himself as the undisputed king of the weight class.
That is until TJ Dillashaw got his shot at UFC gold. Coming off a dominant, though hardly title-inspiring, win over Mike Easton Dillashaw was a huge underdog entering the cage against Barao.
Despite this, the then-Team Alpha Male product delivered one of the most iconic performances in UFC history, barely putting a foot wrong en-route to a fifth-round TKO win.
Event: UFC 193
Opponent: Ronda Rousey
In the same way that Mike Tyson vs Buster Douglas is still talked about today, Holly Holm knocking out Ronda Rousey will forever remain a prominent part of this young sport's folklore.
Despite her decorated background in both boxing and kickboxing, Holly Holm entered the Octagon on November 14th 2015 largely written off by both fans and analysts of the sport as little more than another name to add to the champion's unprecedented MMA legacy.
And while the years that followed have done little to protect Rousey's reputation within combat sports, it is worth remembering just how dominant she had been up to that point. With a world-class grappling game thanks to her Olympic judo background, coupled with powerful, albeit rudimentary, striking skills Rousey was unlike anything anyone had ever seen in women's MMA.
However, despite Rousey's success, it became clear throughout her UFC run that she possessed significant shortcomings within her game, particularly in her striking defence.
When the pair finally met in the centre of the Octagon in front of a record-setting crowd in Australia, Holm would dominate Rousey, utilising her lateral movement and precision striking perfectly, knocking the previously unbeaten champion out in the second round.
Event: UFC 68
Opponent: Tim Sylvia
With nearly a decade having passed since Randy Couture made his final bow in the UFC, it is becoming increasingly clear that an entire generation of MMA fans have missed out on seeing The Natural compete inside the Octagon.
Though he never embarked on a run of dominance like other Hall of Fame greats, Couture left a lasting impact on the sport for his incredible work ethic, and his incredible resume featuring some of the recognizable names in the sport across two weight classes.
However, in 2007 it appeared to many that Couture's time in the sport was coming to an end. After finding himself on the receiving end of a Chuck Lidell knockout to conclude their legendary trilogy, Couture announced that he planned to return to heavyweight to face reigning champion Tim Slyvia.
Despite the massive size difference, Couture lived up to his nickname from the opening bell, felling his giant opponent with the very first punch of the fight and maintaining a frantic pace throughout all five rounds.
When Bruce Buffer announced the judges' decision there could have only ever been one winner as Couture continued his unprecedented legacy by becoming the oldest man ever to win UFC gold, a record that stands to this day.
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