UFC welterweight champion Leon Edwards is set for a legacy-defining test on Saturday when he heads into his trilogy fight against Kamaru Usman as the underdog.

Edwards, 31, suffered the last loss of his career against Usman — a unanimous decision back in 2015 as he failed to solve the wrestling-heavy attack from the 'Nigerian Nightmare'.

It took nearly seven years for Edwards to earn the rematch, rattling off nine wins in a row over that span to force his way into a world title fight.

He got his opportunity at UFC 278 in August, and while he showed some clear improvement from their first meeting – including landing a rare takedown against Usman – the champion looked set to extend his perfect run in the UFC to 15-0 through four rounds.

But just minutes away from a decision victory, Edwards did the unthinkable. After repeatedly throwing his left roundhouse kick to the body and legs throughout the opening 22 minutes, Edwards sent the same kick high, catching Usman clean as he instinctively leaned into it and defended his body.

It was a moment that will live forever, with the man aptly nicknamed 'Rocky' coming from the clouds to score a monumental upset and conquer the fighting world.

The passion, pain and frustration from his decade-long journey to the top was evident during his famous post-fight interview, where he stared into the camera yelling, "You all said I couldn't do it — well look at me now".

Nobody can ever take that night away from Edwards, and people will still be talking about it long after both he and Usman have hung up their gloves — but the reality is that his dream run may be in its final hours as the trilogy approaches.

While Edwards has proved he is capable of defeating Usman — something none of his previous opponents can say — it is still hard to imagine how he can win three out of five rounds against the former champion.

The grappling advantage for Usman is significant, and he is likely to lean into that even further after the painful illustration about what can happen if he settles for a kickboxing match.

Even after Edwards fought off the grappling attack from Usman in the first round of their title fight, Usman almost assuredly took rounds two, three and four, and really looked in no danger down the stretch until the fight-ending blow.

Usman is simply better at winning rounds, meaning Edwards likely has to repeat his knockout finish to defend his belt for the first time in front of a packed O2 Arena.

It would be unfair to label the historic head-kick as a fluke, or luck. You do not accidentally set someone up for a perfect finishing shot and land it with such force, at such a desperate situation in the biggest fight of your life.

But the thing about once-in-a-lifetime knockouts is that, by definition, they don't happen twice — and a fighter the calibre of Usman will not make the same mistake again.