Sports at the highest levels are defined by rivalries, between the athletes as well as between the teams. There was Magic and Bird in the NBA, Arnie and Jack on the PGA Tour, Ali and Frazier in boxing, and the Yankees and Red Sox in baseball.
Until this point, though, the UFC has never had a rivalry that could touch Cain Velasquez and Junior dos Santos.
They'll meet for a third, though hopefully not final, time Saturday at the Toyota Center in Houston in the main event of UFC 166.
The UFC has had a few rivalries of note over the years, some of them contrived, others unquestionably legitimate, but there has never been anything close to Velasquez and dos Santos.
Theirs is a rivalry not borne of a silly feud or a marketing gimmick.
It is entirely because of their immense skill and respect for each other that there is a rivalry at all. These are the two best heavyweights in the world – far and away, the two best heavyweights in the world – and so the stakes are enormous every time they fight.
Dos Santos got the first victory when he knocked out Velasquez in just 64 seconds on Nov. 12, 2011, in Anaheim, Calif., in the main event of the first UFC on Fox show. Velasquez evened the score, and regained the title, when he mauled dos Santos on Dec. 29 at UFC 155.
They're set to fight a third time Saturday, kicking off the UFC's fantastic finish to the year that includes three mega pay-per-view events.
It's a major step forward for mixed martial arts, because high-level rivalries showcase a sport at its best and create long-term fans. It's a little more than 38 years since Muhammad Ali and Joe Frazier met for the third time in a bout known forever as "The Thrilla in Manila," and it's still spoken of in reverential terms today.
Fox and ESPN can't get enough of Yankees-Red Sox because the rivalry between the teams and their fans is always compelling and draws big numbers.
Being a one-on-one sport, the UFC is built for such rivalries. There have been several notable ones, but for numerous reasons, none of them have reached the historic level.
The first rivalry of note was in the UFC's early days, between Ken Shamrock and Royce Gracie. That, though, came when the sport was so young that virtually no one understood much about jiu-jitsu and what was happening.
Several years later, Shamrock and Tito Ortiz met in a three-fight series that felt way too much like pro wrestling and was incredibly one-sided in Ortiz's favor.
Ortiz dominated Shamrock at UFC 40 in a fight that, for the time, did strong pay-per-view business. The pay-per-view sales, and not the competitive nature of the first bout, which ended after two rounds when Shamrock's corner threw in the towel, dictated a rematch.
They met again at UFC 61 with Shamrock a shell of his former self. Ortiz won easily again, but fans were outraged by what they felt was a quick stoppage in Ortiz's first-round victory.
So, UFC president Dana White matched them a third time in a show on Spike TV that, not surprisingly, Ortiz again won with ease.
There was nothing in that rivalry that appealed to casual sports fans. The pro wrestling shtick between them was a massive turnoff to fans who don't want to have to guess whether they're watching a legitimate athletic competition or a "work." And Ortiz was so dominant, there was rarely any question of who'd win.
Ortiz had a legitimate rivalry with Chuck Liddell, but its significance was lessened because Liddell easily won both of their bouts.
There have been other rivalries – Georges St-Pierre against both Matt Hughes and B.J. Penn, Hughes against Penn, Frank Trigg and Matt Serra, Penn against Jens Pulver, Liddell against Randy Couture, and Chael Sonnen against Anderson Silva, among others – but all of them fall short of what the UFC has in Velasquez and dos Santos.
In some of those fights, one fighter was near the end of the line. In others, one had to move up to challenge the other.
None of that exists with Velasquez-dos Santos. They're natural heavyweights in the primes of their careers. Removing fights against each other, their combined record is 26-1.
They're well-rounded, well-conditioned and young. Velasquez, at 31, is 18 months older than dos Santos.
There have been some who have characterized the rivalry as one-sided by referring to dos Santos' victory over Velasquez as a fluke and pointing to a knee injury Velasquez fought with.
Dos Santos, though, was also badly injured going into that bout. Plus, he is a striker, and though his knockout at UFC on Fox didn't come from a classically thrown right hand to the chin, it nevertheless accomplished its aim by ending the fight.
That's the thing with knockout punchers: They have the ability to end the fight at any time. Dos Santos proved he doesn't have to land flush with his best shot to score a knockout, and that makes him dangerous.
Given that, it's unfair to minimize his victory, because he did what knockout punchers do.
Velasquez's wrestling-heavy style led him to a far more brutal and one-sided win at UFC 155. Velasquez took down dos Santos, beat him on the ground and left the Brazilian's face swollen and contorted almost beyond recognition.
Another win of that ilk, with Velasquez dominating much or all of the way, and the rivalry is likely to end. It would be clear in that case who the better fighter is, and usually when the better fighter is definitively determined, a rivalry ends.
But if dos Santos wins, or if Velasquez wins a back-and-forth fight, there is likely to be a fourth, fifth and – who knows? – perhaps even sixth match.
Boxers Sugar Ray Robinson and Jake LaMotta fought six times, and when the fighters are elite and the matches are compelling, there is no reason to quit fighting one another.
Given the state of the UFC's heavyweight division, this probably won't be the last time that Velasquez and dos Santos are locked into a cage with each other.