Hard to believe that just a few short years ago, Mauricio "Shogun" Rua was considered the most feared light heavyweight fighter on the planet, yet when he takes to the cage this weekend at UFC 104 for his title fight against Lyoto Machida, he'll do it as a significant underdog written off by most experts.  

During the glory days of PRIDE, Rua soccer-kicked and head-stomped his way to the world No. 1 ranking in the 205-pound class. He sprinted to 12-1 record in the Japanese promotion, disposing of much of the world's top competition in the process. When that organization fell apart and Rua made his way to America, it was with considerable expectations, perhaps even the assumption that he'd soon have UFC gold around his waist. 

Then came the fall from grace. 

Like a couple of other notable Japan-based fighters, Rua faltered in his Octagon debut, getting choked out by Forrest Griffin in the third round of their fight at UFC 76 in Sept. of 2007. Discounting an injury he suffered against Mark Coleman, it was his first real loss in four years and did as much to undermine Rua's reputation as it did to catapult Griffin to the light heavyweight championship in 2008. 

What followed was a near two-year stretch of inactivity during which Rua had knee surgery and left the once-vaunted Chute Boxe Academy to start his own camp, called Universidade da luta. When he returned to the Octagon, he avenged the previous loss to Coleman at UFC 93, but the fight between two obviously out-of-shape fighters was roundly panned by critics. 

In April, Rua knocked out Chuck Liddell at UFC 97, but that performance also earned him little respect. Liddell was viewed by many as already over-the-hill and the beating by Rua only forced him into at least a temporary retirement. 

So it is with a somewhat underwhelming 2-1 record in the cage that Rua now takes on Machida, the latest most-feared, most-admired, most-sought-after light heavyweight on the planet. 

Their paths to this bout couldn't have been more different. While many view Rua as being rushed into a title shot, Machida had to go 6-0 in the UFC before he got his chance against then-champ Rashad Evans last May. He responded by KOing Evans in the second. 

While "Shogun" is seen to have lost a step, Machida has emerged from the shadows of his own previous career in Japan as the untouchable, undefeated heir to a 205-pound throne that once belonged to Rua. He's dominated everyone he's been in the cage with, using a distinctive blend of traditional Karate and modern striking techniques to become an elusive and – at least in his last couple of fights – effective finisher. 

Meanwhile, doubts surround Shogun. People say he's too small to fight at 205-pounds in America, where many fighters cut large amounts of weight to make the limit. People say multiple knee injuries have robbed him of his natural explosiveness and his killer instinct. People say he can't excel under the UFC's unified rules, which stripped him of the kicks and stomps he used so effectively in Japan. 

The former PRIDE champion will enter the cage as one of the longest long-shots on the pay-per-view card this Saturday night. If he wins, it will constitute one of the better comeback stories you'll see in MMA this or any year. If he loses, as the oddsmakers predict he will, it's difficult to forecast what the future may hold for a veteran fighter who could be considered washed-up at 28-years-old. 

No matter what happens, Rua's story serves as a stark reminder of how thin the margin for error is in this sport, how narrow the gap between best-ever and also-ran. If he manages to hoist the gold, the previous two years of setbacks and unfulfilled promise will be forgotten in an instant. If he falls short, he'll sink deeper in the eyes of a public, who once loved him and now view him as just another victim for their newest hero.