Holly Holm kicks Ronda Rousey in the head, resulting in a knockout victory. Photograph by Jeff Bottari / Zuffa LLC via Getty

T_he Ring_ is a boxing magazine that has been published since 1922, but for the cover of its January, 2016, issue, its editors chose a champion from a different sport: Ronda Rousey, the Olympic judo medalist who has become perhaps the biggest star in the history of mixed martial arts. “She conquered M.M.A.,” the headline said. “Is boxing next?”

The two sports are similar enough to be natural rivals: boxing is old-fashioned and single-minded, bearing the sometimes oppressive weight of its distinguished history; M.M.A., which was codified in the nineteen-nineties, is modern and proudly hybrid, borrowing techniques (and sometimes athletes) from Brazilian jiu-jitsu, wrestling, judo, and other disciplines, including boxing. Many M.M.A. fights end with a decision, or a submission, but at the time the cover appeared Rousey had won two of her last three fights by knockout; she was the undefeated U.F.C. bantamweight champion, dominating her division so completely that it seemed reasonable to wonder whether she might be ready for a new challenge, just to keep herself from growing bored.

On Sunday morning, in Melbourne, Australia (Saturday night in America) Rousey was the headliner at U.F.C. 193, facing an opponent named Holly Holm, who seemed likely to test Rousey’s boxing ability. Holm was a boxing champion who had converted to M.M.A. in 2011, winning her first nine fights, though not always easily. Promotional material from the U.F.C. portrayed Holm as brutal striker, and therefore a true threat to Rousey, but the oddsmakers were not so impressed: as the pay-per-view broadcast began, Rousey was a twelve-to-one favorite. And many fans probably bought the fight mainly to find out whether Holm—an earnest and soft-spoken fighter who calls herself The Preacher’s Daughter, in tribute to her father, a pastor in the Church of Christ—would become the fourth straight Rousey opponent to be destroyed in less than a minute.

There had been some last-minute excitement when the fighters, upholding a venerable tradition, exchanged words and shoves at the weigh-in. The (https://www.youtube.com/watch?v=JKGvMrqQH10#t=35m30s) appeared to show Holm pushing her right fist into Rousey’s face, although a different angle revealed that, in fact, Rousey had pulled Holm’s fist toward her. Rousey, known for her pre-fight scowl, likes to find a way to view her opponent as her enemy, and in the aftermath of the weigh-in, she took to Instagram to make it clear that she had found one:

Fake ass cheap shotting fake respect fake humility bitch - “preacher’s daughter” my ass - I see through your fake sweet act now - you’re getting your ass kicked tomorrow, and I’m really going to enjoy the beating I give you #andSTILL

The fight began with Rousey moving forward and Holm moving backward, which was what most people expected. And yet Holm, stepping nimbly away from Rousey, looked surprisingly comfortable. At one point, Rousey (who fights from the orthodox stance) was chasing Holm (who fights from the opposite stance, southpaw) around the ring, clockwise, when Holm suddenly stopped and pelted Rousey with a three-punch combination. As a judoka, Rousey likes to grapple, but when she caught Holm in a clinch, a minute later, Holm got away, delivering a right hook and a left kick as she escaped.

Already, Holm, the former professional boxer, was making Rousey’s boxing skills look distinctly nonprofessional. While Holm darted and slipped, Rousey moved more predictably, standing stiff and erect, which made her head a relatively easy target for Holm’s left fist. Joe Rogan, the sharp U.F.C. analyst, began to sound the alarm. “Ronda looks exhausted,” he said. “Hands are down, chin is up.”

Mike Goldberg, the play-by-play announcer, alluded to Rousey’s growing celebrity: she has a wide range of interests and opportunities besides fighting, including a burgeoning film career. “It takes a lot of energy to be a rock star,” he said.

Rogan was more alarmed. “She’s getting punched in the face,” he said. “It has nothing to do with being a rock star—she’s getting lit up!”

But why? In M.M.A., a common solution to getting punched in the face is to pull or flip your opponent onto the mat; this would seem to be especially important if you are trained in judo and your opponent is trained in boxing. But between rounds, Rousey’s coach, Edmond Tarverdyan, seemed surprisingly unconcerned that Rousey surely had, for the first time in her M.M.A. career, lost a round. Tarverdyan had previously expressed his certainty that Rousey was a better boxer than Holm, and now he was encouraging her to keep boxing. “Champ, beautiful work,” he said. “All she wants to do is catch you with that left hand and come on top with that hook, O.K.? We’re feinting. We’re keeping both hands up.”

Holm’s coaches, Greg Jackson and Mike Winkeljohn, exuded confidence, too, though they had good reason. “She’s going to get a little more desperate this round,” Jackson said. Sure enough, Rousey began to flail. She threw a left hook and Holm ducked; the punch missed so wildly that Rousey landed on one knee, with Holm safely behind her. Rousey was already starting to look like a beaten fighter, muddled and bloody mouthed, when, with less than a minute elapsed in Round Two, Holm punched Rousey in the chin and threw her down. As Rousey staggered back to her feet and prepared to reëngage, Holm delivered her left foot to the right side of Rousey’s face, creating one of the most memorable images in the sport’s short history: Rousey collapsing, limp, onto her left side, where she absorbed a few more punches before the referee could rescue her. Rogan’s spur-of-the-moment verdict may well hold up: “The greatest-ever upset in U.F.C. history.” Holm celebrated and then returned to the spot of the knockout, where Tarverdyan and a cluster of officials were helping Rousey to sit up. Holm watched the scene, as if she were still trying to convince herself that Rousey no longer posed any threat to her.

It was an extraordinary night for Holm, and in some senses a simple one: she won the biggest fight of her life, and she is suddenly not only an M.M.A. champion but a hero, too. But for the U.F.C., the situation was more complicated: this was one of the most memorable fights in the organization’s history, but it also dimmed the sport’s biggest star. At the post-fight press conference, Dana White, the U.F.C. president, struck an optimistic note. “These are the moments in fighting that make it so crazy, and so fun,” he said. And he acknowledged that a rematch—which would be a guaranteed blockbuster—“makes a lot of sense.” It is now clear that Rousey is beatable; the question is whether that makes fans more or less eager to watch her fight.

Assuming, that is, that Rousey herself remains eager. Unlike her peers in M.M.A., Rousey has an entertainment career to consider, although that career is largely based on the perception of her as a conquering hero. Before Holm knocked her out, Rousey had said that she didn’t want to fight again until next summer. And in that post-fight press conference, White confirmed that Rousey had been “transported”—transported, that is, to a hospital, for a post-knockout evaluation, and for some stitches to repair her lip, which was split. White described her as “completely bummed out and depressed.” On Sunday night, she returned to Instagram, telling fans, “I appreciate the concerns about my health, but I’m fine. As I had mentioned before, I’m going to take a little bit of time, but I’ll be back.”