It was a dominant performance by the reigning rock star of MMA. Davis, ranked second among 135-pound contenders, literally didn't know what hit her.
Rousey hurt Davis with a punch to the face at the centre of the cage, punished her with a knee then took her down with a hard judo throw, with Montreal referee Yves Lavigne stepping in seconds later after Davis absorbed some 10 rapid-fire blows to the face.
A stunned Davis had no idea what had happened as she clung to the ref after the stoppage before a Mandalay Bay Event Center crowd of 10,088 that hardly had time to take its seats.
"Obviously a lot going through my head right now," Davis said at the post-fight news conference. "Well, what are you going to do. There's a lot of stuff that I didn't do that I wanted to do ... But live and learn from your mistakes. Just get back up and keep fighting."
The 16-second knockout is tied with Frank Shamrock's win over Kevin Jackson for second fastest in a UFC championship fight. Andrei Arlovski's 15-second KO of Paul Buentello is the fastest.
Rousey was promptly offered the UFC 176 main event on Aug. 2 — featherweight champion Jose Aldo recently pulled out due to injury — to which she said in the cage that she would consult her coaches and go ahead with their say-so. UFC president Dana White said later that he had no idea why the offer was made, calling it "the biggest idiotic move in the history of our production team."
Rousey will need time off to heal, despite the short outing.
She had nine stitches near her knuckle on the first finger of her right hand after the fight, the result of a cyst that had caused the skin on her knuckle to come loose after her last few fights. The champion also said she will need knee surgery down the line to correct a chronic issue.
Rousey did say she was open to fighting on the UFC's New Year's card.
The 27-year-old Rousey came into the fight as a 10-1 favourite to beat Davis, a 29-year-old from Port Colborne, Ont., who fights out of San Jose.
'If the co-main event was short and violent, the main event was 25 minutes of sweat, strategy and stamina.
Middleweight champion Chris Weidman needed to dig deep to dispose of former light-heavyweight title-holder Lyoto (The Dragon) Machida via a 49-45, 48-47, 49-46 decision.
"He's as good as I thought," Weidman said of Machida.
Asked if he was ever hurt in the fight, the champion replied: "Probably. I can't remember right now."
He said later he had been unable to hit the last two weeks because of hand issues, acknowledging his camp had been fraught with problems.
In the past, Machida has been accused of being a boring fighter. But the Brazilian gave Weidman everything he could handle and aggressively went for the finish late after falling behind early.
He was rewarded with loud cheers after the fight.
"He's a true champion," Machida (21-5) said of Weidman, who had never gone five rounds before.
The battered Brazilian smiled and held his fist up as he walked out of the cage. Weidman left wrapped in a U.S. flag.
Weidman, who came in as a 2-1 favourite, looked much bigger in the cage than Machida and stalked him in the first round. The elusive Machida dodged blows and looked to counter-attack but Weidman kept coming.
Weidman (12-0) scored a takedown late in the second round, smothering Machida while doing damage at the fence. When the Brazilian got up, he ate a knee.
Machida went down again in the third, to cheers from the pro-U.S. element of the crowd. Weidman cut him with a punch late in the round and took him down again before bodyslamming him to the canvas.
Machida, to his credit, kept getting up.
The Brazilian nailed Weidman with a body kick in the fourth round, only to have the champion make a "Bring it on" gesture. Machida, knowing he needed something big, started coming forward and connecting.
Both men showed damage from an electric, bruising round that had the crowd chanting "Machida." Weidman's rib cage looked like Machida had taken a baseball bat to it.
The fifth round was equally brutal. Machida survived a head kick and the two tired fighters kept punching. A Weidman takedown was followed by a wild Machida flurry as the fight ended.
Machida, his face ravaged by ugly red abrasions and bumps, held an ice pack on his head at the post-fight news conference.
"I'll be back better," he said.
It was Weidman's first outing since back-to-back wins over Anderson Silva.
Weidman's basic purse was listed at US$225,000 with a matching win bonus, according to the Nevada State Athletic Commission. Machida was on a basic $200,000 purse.
The two also got an extra $50,000 each for fight of the night.
Rousey's purse was listed at $60,000, with a matching win bonus, and she picked up another $50,000 for performance of the night. Davis was on $24,000, with a matching win bonus.
The commission figures do not tell the whole story, however, since the UFC does not detail all the details of its fighter compensation.
Rousey, with her fourth title defence, improved her record to 10-0 while Davis fell to 16-6. The champion finally smiled in the wake of her lopsided win, with the crowd cheering her on.
It was a matchup of the only female fighters with 3-0 UFC records.
A smiling Davis, who has black belts in Brazilian and Japanese jiu-jitsu, entered to "Royals" by Lorde. Husband Flavio Meier, a BJJ black belt who trains and corners her, wasn't far behind.
A stone-faced Rousey, an Olympic bronze medallist in judo, followed to Joan Jett's "Bad Reputation."
There was drama on the card backstage when Dutch heavyweight Stefan (Skyscraper) Struve, slated to make a comeback after a heart scare, fainted backstage and was scratched from his bout with Matt Mitrione.
White said Struve passed out while warming up. When he came to, he had an elevated heart rate. White said doctors initially thought it might be a panic attack.
"With his health and safety in mind, he's been removed from the card and is currently under the care of the medical staff," the UFC said in a statement.
The six-foot-11 Struve's MMA career appeared over last year when he was diagnosed with a rare heart condition. He was given the OK to resume fighting by his doctors in the Netherlands and the UFC's cardiologist.
The night started with wins by five straight underdogs before veteran bantamweight Urijah (The California Kid) Faber stopped Alex (Bruce Leroy) Caceres via third-round submission in the featured undercard fight.
Faber, a 35-year-old former WEC featherweight champion who has never lost a non-title fight, was a 10-1 favourite.
Caceres, 26, had won four of his last five with another victory overturned due to a positive test for marijuana.
Caceres' length and unorthodox style was making life difficult for the five-foot-six Faber, who scored takedowns but failed to do much damage. But the veteran came out strong in the third, driving Caceres back across the cage with a flurry of blows and then grabbing a leg to tip him over.
Caceres (10-6 with one no contest) gave up his back and Faber (31-7) finished him off by rear-naked choke at 1:09 of the third round.
Faber, who thought he might have broken a rib in the first round, is ranked second among 135-pound contenders while Caceres is No. 12.
Bantamweight Russell Doane (14-3) opened the pay-per-view portion of the card with a 29-28, 28-29, 30-27 split decision win over Marcus (The Bama Beat) Brimage (6-3).
Middleweight Uriah (Prime Time) Hall survived a gruesome toe injury to win a 29-28, 29-28, 30-27 decision over Brazil's Thiago Santos.
Hall, who has been called out by White for not delivering on his skills in losing his first two UFC fights, showed his grit by fighting on with a broken toe from the first round. Hall's toe was pointing up as the second round started. The doctor spoke to Hall after the second round but allowed the fight to continue.
Every time the bent toe was shown on the big screens, the crowd groaned. Especially post-fight when a shot showed the bone protruding.
"Any movement I would feel my bone shift in and out of the skin," said Hall.
To add insult to injury, a limping Hall was kicked in the groin in the third round.
Hall, who limped out of the ring holding onto a cornerman, received thunderous applause after the decision was announced in his favour. He later tweeted an X-ray of the busted toe.
Earlier, middleweight Luke Zachrich (14-3) won a 30-27, 30-27, 29-28 decision over Brazilian TUF alumnus Guilherme (Bomba) Vasconcelos (3-2).
Middleweight (King) Kevin Casey (9-3) spoiled the Octagon debut of Bubba (The Fighting Texas Aggie) Bush (8-3), putting him down him with a left hook before finishing him off with a string of elbows on the ground for a 61-second TKO.
Bantamweight Rob Font (11-1) enjoyed a spectacular debut, hammering George Roop with a right to the head that literally had the six-foot-one Roop (15-2-1) out on his feet. Roop buckled and then fell to the ground, with referee Chris Tognoni stepping in at 2:19 of the first round.
Font, who spent some three years delivering pizzas, also got a $50,000 performance of the night bonus.
Brazilian middleweight Bruno Santos (14-1), a five-foot-nine slab of muscle, evened his UFC record at 1-1 with a 29-28, 28-29, 29-28 split decision over Chris Camozzi (19-8).
Welterweight Kenny Robertson (14-3) won a unanimous 30-26 decision over Brazilian Ildemar Alcantara (20-7).
Saturday's card was part of International Fight Week in Las Vegas, a glut of MMA offerings that concluded with a weekend Fan Expo and two UFC cards: UFC 175 and Sunday's finale to Season 20 of "The Ultimate Fighter."