As has been widely noted by Joe Frazier’s eulogists, it’s almost impossible to discuss Frazier apart from Muhammad Ali. What two athletes are as closely linked in the public imagination? Maybe Russell and Chamberlain. Conceivably Magic Johnson and Larry Bird.

Yet despite their joint identity, Frazier and Ali differed at the core. Ali, an unusual mixture of depth and flim-flam, out-talked Howard Cosell, out-promoted Don King, out-lasted the U.S. government. He was a walking advertisement for himself, charisma basted in blarney. It’s easy to picture Ali as a traveling salesman, magician, or carnival barker. By contrast, it’s hard to picture Frazier outside the ring. A no-nonsense man, he seemed born to punch and be punched – a prizefighter’s prizefighter.

They were fundamentally different inside the ring as well, even beyond the stylistic divide: Frazier plowing ahead and unleashing lethal hooks, Ali back-pedaling or dancing side to side while tossing jabs. Ali improvised the rope-a-dope to defeat George Foreman; Frazier knew only one way to fight. The two evoke Odysseus and Ajax, who in the proto-Olympic games in the Iliad, engage in a spirited wrestling match. Odysseus, while a great athlete, cannot match the overpowering Ajax in sheer physical prowess, but his resourcefulness counteracts the other’s power. They wrestle to a draw and split the prizes, not unlike Ali and Frazier, whose three fights, taken as a unit, produced no clear winner but cemented their mutual legacies.

For all the differences, Ali and Frazier shared one striking similarity. Everyone remembers that Frazier decked Ali with a thunderous hook in the 15th round of their first fight; many forget that Ali bounced off the canvas and actually had the better of the action in the fight’s final minutes. Can you imagine the toughness required to do that? Frazier, for his part, was so battered by the fight that he needed a prolonged hospital stay to recover. But, in the ring, he never rested. That’s what united the arch-rivals: will.

An unfortunate defining moment in Frazier’s career is his six knockdowns at the hands of George Foreman, made famous by Howard Cosell bellowing, “Down goes Frazier!” The thing to remember, though, is up popped Frazier – six times. Frazier and Ali both absorbed some frightful beatings, but neither ever went down for the count. An enduring image from their three bouts is Frazier’s trainer, Eddie Futch, refusing to allow his man to go out for the final round in their final fight. Frazier, a dead man walking, protested the decision. Ali, almost as beaten up, later declared that he felt like death. Someone else had to stop the fight, because these two wouldn’t. That mutual indomitability, as much as anything else, is what made the Thrilla in Manilla so compelling.

Champion prizefighters are a pretty tough lot, but the combination of the physical pounding and emotional travail drives them to the breaking point surprisingly often. Roberto Duran was a fearless street-fighter, but famously said “no mas” against Sugar Ray Leonard. Sonny Liston arguably twice tanked it against Ali. Mike Tyson had a number of moments where courage gave way to some pathology or other. There is a long list of boxing greats who at one time or another showed a failure of fortitude. Ali and Frazier never made the list; the very idea is hard to imagine.

Many remark Ali’s infectious cheerfulness and Frazier’s dignity, but let’s not overlook the virtue they shared: an almost superhuman toughness.