Few sane experts believed Matt Serra could pull off the unthinkable and dethrone welterweight champ Georges St-Pierre at UFC 69, especially after "The Terror" narrowly edged Chris Lytle in the finale of the The Ultimate Fighter 4: The Comeback.
Lytle not only outstruck Serra 195-91, including 42-20 in the significant strikes category, he also scored on one of two takedown attempts and stuffed 15 of 16 shots from the New York native.
Nonetheless, Serra got the nod from two of the bout's three judges, earning a split decision win over Lytle and an opportunity to square off with reigning champ GSP at UFC 69.
Deemed a massive underdog, Serra stunned St-Pierre and the MMA community by scoring a TKO win at 3:25 of the first round.
Although several underdog stories have emerged in the sport since, Serra's miraculous upset represents the most notorious and mind-boggling in UFC history.
Here are the 10 greatest true-to-life Rocky-esque stories in MMA history.
Costa Philippou certainly didn't look like a future middleweight contender in the UFC when he dropped his preliminary-round bout against Joseph Henle on The Ultimate Fighter Season 11 premiere.
If anything, Philippou portrayed an extremely raw talent with a gift for striking and a weakness for grappling.
Henle absorbed a mauling for the better part of two rounds before hooking up a fight-ending armbar to keep Philippou from becoming a TUF cast member.
Fortunately for UFC president Dana White, he and his team had the foresight to sign the lethal Philippou and pass on journeyman Henle.
While he's experienced success in mid-tier promotions following his stint on The Ultimate Fighter, Henle suffered a brutal loss to Elvis Mutapcic (leg kicks) in the Maximum Fighting Championships in October.
Philippou, conversely, has piled up five straight wins in the UFC since dropping his opener to Nick Catone (unanimous decision) at UFC 128.
Philippou scored the most significant victory of his fledgling career when he TKO’d Tim Boetsch at UFC 155, a win that catapulted the 33-year-old to No. 6 on the UFC's middleweight rankings.
Setbacks to Tito Ortiz and Keith Jardine, among other variables, made the original winner of The Ultimate Fighter, Forrest Griffin, a heavy underdog entering his bout with former Pride champ Mauricio "Shogun" Rua at UFC 76.
But like he's done before and since, Griffin defied the odds and valiantly finished Shogun via rear-naked choke, setting up a bout with Quinton "Rampage" Jackson for the light heavyweight strap at UFC 86.
Once again reveling in his underdog role, Griffin outstruck Rampage 113-73, including 77-50 in the significant strikes department, leaving with a unanimous decision win and the light heavyweight belt.
Griffin announced his retirement in May at the age of 33, four years after writing the first of a pair of New York Times bestsellers.
Shortly after his retirement, White announced that Griffin, alongside sidekick Stephan Bonnar, will represent the next two fighters inducted into the UFC Hall of Fame.
Less than nine months after breaking his femur and tearing several ligaments in his knee in a brutal motorcycle accident, the UFC stripped Frank Mir of his heavyweight belt.
Doctors couldn't guarantee that Mir would ever run again, let alone fight in the UFC.
In February 2006, Mir had finally recovered enough to return to action, although he disappointingly dropped two of his first three bouts in the UFC. Mir lost via TKO to heavy underdog Marcio Cruz at UFC 57 before getting TKO'd by Brandon Vera at UFC 65.
All but counted out by fans and experts, Mir made a triumphant comeback and won his next three fights, including an upset TKO victory over Antonio Rodrigo Nogueira at UFC 92, a win that once again made Mir heavyweight champ.
Mir, who's still a viable contender nearly nine years after his accident, went on to compete in an interim title fight against Shane Carwin at UFC 111. He then fell to Junior dos Santos with the heavyweight belt up for grabs at UFC 146.
Only forgettable losses in a cluster of title fights kept Mir from landing in the top five of this countdown.
UFC president Dana White nearly passed on one of most popular and entertaining fighters in his stable in hard-hitting heavyweight Mark Hunt in 2009.
White initially didn't think Hunt had the chops to dance with the UFC's top-tier heavyweights, and rightfully so.
A former standout in K-1, Hunt went on a four-year winless drought that spanned from July 2006 to February 2011. In that stretch, Hunt dropped six straight bouts, amazingly losing five by submission and another by KO.
So instead of getting the call to fight in the UFC in the midst of a winning streak, like most up-and-comers in the company do, Hunt got his opportunity from White through a technicality.
Because of strings attached to Zuffa's deal with Japan's Pride Fighting Championships organization, Hunt could have just collected a bulky paycheck from White without ever fighting in the UFC.
But five losses into his losing streak, Hunt demanded a fight, a request matchmaker Joe Silva made a reality when he paired "The Super Samoan" with Sean McCorkle at UFC 119.
After losing to McCorkle, Hunt reeled off four straight wins in the UFC, three of which came via form of knockout.
Hunt recently absorbed a Junior dos Santos knockout from a spinning heel kick and punches at UFC 160, a setback that prevented him from climbing higher on this list.
The loss, however, didn't diminish the remarkable Cinderella story that Hunt's pulled off in his three years in the UFC.
Born with a congenital amputation in his left arm, Nick Newell learned to operate with one arm from the get-go in his life.
Against all odds, Newell began a pro MMA career in 2009, astoundingly prevailing in his first five bouts before joining the XFC promotion in 2011.
Newell racked up four straight wins in the XFC, two of which came via submission and another by knockout.
Newell's most notable win came against Bellator MMA veteran Eric Reynolds in his last fight at XFC 21. Newell sunk in a rear-naked choke and finished Reynolds just 1:22 into the fight.
For his efforts, Newell inked a multi-fight contract with the World Series of Fighting on May 28, a break that could change his fortunes for good.
From his days on the original season of The Ultimate Fighter as a middleweight, Kenny Florian perpetually made a name for himself by proving doubters wrong.
Florian rode the underdog role all the way to the finals of the first season of The Ultimate Fighter, where he lost via TKO to Diego Sanchez.
Still tabbed a bottom dweller following his loss to Sanchez, Florian relished being the underdog and won his next three fights to get a crack at Sean Sherk and the lightweight belt at UFC 64.
After losing to Sherk, Florian won nine of 12 fights in the lightweight and featherweight divisions, earning title shots against both B.J. Penn (lightweight) at UFC 101 and Jose Aldo (featherweight) at UFC 136.
Named as large as a 5.5-to-1 (+550) underdog before his first encounter with B.J. Penn, Frankie Edgar pulled off the unthinkable and upset "The Prodigy" to garner the lightweight belt at UFC 112.
Still an underdog in the rematch, "The Answer" made things look easy and outstruck Penn 142-56 en route to another unanimous decision win at UFC 118.
After putting his rivalry to rest with a KO of Gray Maynard in their third fight at UFC 136, "The Answer" seemingly began another grudge match with a bigger and more physical 155-pounder in Benson Henderson.
In back-to-back contentious wins, "Bendo" captured the belt with a unanimous decision win over Edgar at UFC 144. Henderson then to defended his strap with a controversial split decision win against Edgar at UFC 150.
Edgar dropped a weight division to challenge seemingly unbeatable featherweight champ Jose Aldo at UFC 156. "The Answer," however, experienced a taste of déjà vu and got edged on the scorecards in a unanimous decision loss to the slightly favored Aldo.
For the first time since his bout with journeyman Matt Veach at The Ultimate Fighter Season 10 Finale, Edgar will get the oddsmakers' blessings as the favorite against Charles Oliveira in his next fight at UFC 162.
It didn't matter that Randy Couture waited until shortly before his 34th birthday to make his pro debut. It also held little weight that Couture's first pro opponent, Tony Halme, outweighed him by nearly 100 pounds.
Truth be told, pundits nicknamed Couture "The Natural" for good reason.
Couture set the tone for a long career of overachieving when he choked Halme in just a minute at UFC 13.
Two bouts later, and following an upset TKO win over Vitor Belfort, Couture used his wrestling chops to beat Maurice Smith for the heavyweight strap at UFC Japan, marking the first of a record 15 appearances in UFC title fights.
Couture became the first fighter in UFC history to capture belts in different weight classes by besting light heavyweight champ Tito Ortiz at UFC 44. But the defining moment of the Natural's career didn't truly occur until the former Oklahoma State University wrestling standout turned 43.
Following back-to-back losses to Chuck Liddell in light heavyweight title tilts, Couture emerged from a temporary retirement to challenge gargantuan heavyweight champ Tim Sylvia at UFC 68.
In a bout few believed he could win, The Natural outstruck the 6'8" Sylvia 138-39 and scored on seven of his 10 takedown attempts. The win made Couture the oldest champ in UFC history.
The UFC's original underdog, Royce Gracie represented his family with tremendous pride in the early days of the company—despite the notion that his half-brother Rickson seemingly possessed the better skill set.
Regardless, Royce Gracie landed at No. 2 on this list because of his propensity for taking out bigger, faster and stronger athletes for the bulk of his career.
Although Gracie dealt with size, strength and athletic disadvantages in each of his first 11 fights in the UFC, the Brazilian Jiu-Jitsu master still amazingly won each of those bouts by submission.
Gracie didn't taste defeat in the Octagon until making his unceremonious return against former welterweight kingpin Matt Hughes at UFC 60.
Gracie absorbed a brutal beating at the hands of Hughes and suffered just the second setback in his career.
Japanese legend Kazushi Sakuraba, who Gracie later defeated, handed the Brazilian his first loss at Pride Grand Prix 2000 Finals.
Of the 16 fighters featured on The Ultimate Fighter 4: The Comeback, Serra, who previously amassed a 4-4 record in the UFC, didn't even stand out as a favorite to win the show.
And considering the lackluster performance Serra put on in his win over Lytle in the season finale, and St-Pierre's dominant win over Matt Hughes at UFC 65, "The Terror" deservedly entered his title fight at UFC 69 a significant underdog.
In the most lopsided title fight in UFC history, at least on paper, St-Pierre, who enjoyed an eight-inch reach advantage in the fight, was named an 11-to-1 favorite (-1100) while Serra was deemed a 7-to-1 underdog (+700).
Despite the extraordinary odds stacked against him, Serra didn't waver, clipping St-Pierre with a looping right hand just over three minutes into the fight to begin a shocking series of events.
A wobbled GSP tried to escape a barrage of punches from the swarming Serra, only to fall to his hands and knees and eventually succumb to a TKO loss at 3:25 of the first round.
St-Pierre's convincing win over Serra in their rematch at UFC 83 didn't do much to tarnish "The Terror's" legacy. Truth be told, Serra's name will always ring synonymous with underdog—at least until someone trumps his utterly stunning win over St-Pierre at UFC 69.