When Anderson Silva steps in the cage this weekend against Chris Weidman, he will look to keep his seven-year title reign intact. Though a Weidman win would hardly be the most shocking in the history of UFC championship fights, it would be an improbable one, considering Silva is (probably) the greatest fighter in the history of the sport.
But Weidman wouldn't be the first to pick up an unlikely win against a dominant fighter and champion.
Other champions have set foot in the cage with the odds in their favor, only to witness the belt change hands at the end of the night. So here are the top five most improbable title wins in UFC history.
Note: This list will include only upsets which occurred during the Zuffa era, meaning Maurice Smith defeating Mark Coleman is out of the picture.
Let's get the obvious one out of the way first.
At UFC 69 back in 2007, Matt Serra shocked welterweight champion Georges St-Pierre with a first-round technical knockout win. The victory earned Serra the title.
It was also the most improbable title win in the history of UFC and, probably, the history of the sport. But you should already know that, so we'll make this short.
Serra would go on to lose that title at UFC 83 a full year later without having defended the title belt. And he would never again fight for the 170-pound strap, or any other championship for that matter.
He was never really considered the best welterweight in the world (even when he held the title), which was displayed by St-Pierre entering the rematch as a 5-1 favorite. But Serra owns a little piece of history after the biggest upset in championship history.
Randy Couture entered UFC 68 as a 2-1 underdog, which is right around where Weidman will enter UFC 162. Those odds suggest the fighter is not expected to win, but it wouldn't be improbable.
But given the fact that Couture was 43 years of age on fight night, this was an unlikely win indeed.
Couture managed to top champion Tim Sylvia in the main event of the card, thanks largely to his incredible game-planning prowess. The fight went for the full 25 minutes, and Couture saw his hand raised in the end.
The UFC Hall of Famer would go on to defend the title just once before running head-on into Brock Lesnar.
After losing the title, Couture would never again fight for one, but his legendary status had already been cemented. He and the UFC may be butting heads at the moment. His place in the promotion's history is set, however.
And the UFC 68 victory played at least some part in punching his ticket to the Hall.
He was small, even for a lightweight, but Frankie Edgar was a fast, efficient striker. At UFC 112 against the great B.J. Penn, it was enough for "The Answer" to steal away the 155-pound strap.
Edgar entered the contest in front of the Abu Dhabi crowd as the obvious underdog—a 5-1 underdog, in fact. That didn't stop him from taking home the championship on points. Penn was stunned by the unanimous decision loss, and "The Prodigy" was given an immediate rematch.
Unlike the aforementioned Serra, Edgar made good in the second meeting, once again earning a unanimous decision.
He would never go on to be as dominant a champion as Penn, finishing his stint at lightweight with just one win in four title fights.
Now at featherweight, Edgar looks to lose the size disadvantage and earn a title once again, though he failed against Jose Aldo at UFC 156.
Regardless of what the future holds, Edgar was the man to derail Penn, who seemed almost unbeatable at lightweight. Better yet, he turned an improbable win at UFC 112 into a competitive run at the top of the division.
Forrest Griffin was not supposed to defeat UFC light heavyweight champion Quinton "Rampage" Jackson at UFC 86. He did it anyway.
The winner of the original season of The Ultimate Fighter, Griffin stunned Jackson with a unanimous decision victory to take home the belt. Thanks to leg kicks and the ability to recover after a rocky first round, Griffin became more than just a winner of the reality show.
But his run as champion was short-lived.
In his first title defense, Griffin was knocked out by Rashad Evans in brutal fashion, something most thought "Rampage" would do to him first.
The fan favorite would never again fight for a UFC title, finishing his career with a 3-2 stretch, including knockout losses to Anderson Silva and Mauricio "Shogun" Rua.
Now retired, Griffin is on his way to a presumably cozy desk job with the UFC. But he's been in more than enough battles to deserve it.
The upset over Jackson was just a nice addition to his Hall of Fame career.
Looking back, it's hard to imagine B.J. Penn was ever an unlikely champion. But he was.
At UFC 46, Penn met welterweight champion Matt Hughes in a five-round affair. "The Prodigy" entered the contest as a heavy underdog, weighing in at plus-300 on the betting line.
He needed less than one round to provide value for anyone who took a flyer on him.
At 4:39 of the opening frame, Penn tapped Hughes, earning the improbable win and taking home the welterweight title.
Following the win, Penn signed with K-1, creating a riff between the UFC and him. He was stripped of the title and would never go on to earn the 170-pound strap again.
But the victory for Penn momentarily derailed Hughes, while also proving that "The Prodigy" was a force at both lightweight and welterweight.
He's one of the better pound-for-pound fighters in the sport's history, and his upset win over Hughes back in 2004 went a long way in proving just that.
Sylvia was previously mentioned for his loss to Randy Couture at UFC 68, but the massive heavyweight had a major upset of his own in the UFC.
At UFC 59, Sylvia was given a rematch against Andrei Arlovski for the heavyweight title—He lost the first meeting via quick submission.
Sylvia took advantage of his second opportunity, overcoming the large odds against him to earn the first-round stoppage.
He would go on to defeat Arlovski in the third meeting in the UFC, while their fourth meeting (which took place outside of Zuffa) ended in a no-contest.
The reason Sylvia's upset of Arlovski doesn't make the list has a lot to do with strength of competition and what we've seen from them since. Neither are UFC quality; neither has been in years.
It would be a long shot for either to return to the sport's top promotion, and even at their prime, Sylvia and Arlovski would struggle against elite UFC heavyweights.
The victory for the poorly nicknamed "Maine-iac" was improbable, but so were his chances of joining a list featuring B.J. Penn, Frankie Edgar and the like.