Alistair Overeem is declared the winner by technical knockout over Brock… (Eric Jamison / Associated…)
Reporting from Las Vegas -- Brock Lesnar could not make it through the first round against Alistair Overeem, and he said the end of the fight also was the end of his mixed martial arts career.
The former Ultimate Fighting Championship heavyweight champion, who came from the World Wrestling Entertainment circuit, was knocked down by two hard right knees and a vicious body kick by former Strikeforce heavyweight champion Overeem.
On the ground, Lesnar endured 10 unanswered punches, the last of which was a loaded-up cannonball right hand that led referee Mario Yamasaki to give Overeem the technical knockout at the 2-minute 26-second mark of the first round at the MGM Grand Garden Arena.
Overeem's victory sets up his next fight to be a heavyweight title shot against champion Junior dos Santos. For Lesnar, 34, it was a final chapter, although there have been indications that he will rejoin WWE in some capacity.
"Hats off to Alistair Overeem," said Lesnar, who was fighting for the first time since October 2010 after suffering through a second battle with the painful intestinal disease diverticulitis. "I've had a really difficult couple of years with my disease.
"I'm going to officially say tonight is the last time you'll see me in the octagon. I want to thank everyone. Brock Lesnar is officially retired."
Lesnar told the crowd he divulged to his wife and children that his UFC career was at a crossroads before fighting Overeem.
"I had no idea he was going to do that," UFC President Dana White said of his organization's biggest star's decision to retire. "Brock Lesnar's made a lot of money in his career. What that man accomplished in a short period is amazing."
Lesnar's exit marks an entertaining four years in the sport. Lesnar first caught the attention of UFC owners in a 2007 fight at the Los Angeles Memorial Coliseum.
He was signed by UFC in 2008, and he won the heavyweight title by defeating MMA legend Randy Couture later that year. Lesnar surrendered the belt last year in Anaheim, losing by first-round TKO to Cain Velasquez.
"I promised if I won, I'd go for the title shot, and if I lost …" he said, his voice trailing off. "You've been great."
Overeem, a powerful striker, maintained the advantage as the fight remained as a stand-up battle early, with Overeem dodging Lesnar's kicks and then an attempt by the former NCAA wrestling champion to grab Overeem's right leg.
Overeem avoided a takedown with the escape, and he set up the finish.
"I tried to rely on my defense," Overeem said after what was officially his UFC debut after the company bought Strikeforce this year. "Brock is an exceptional wrestler, so I had to step up my game."
In the co-main event, Stockton's Nate Diaz put on a boxing clinic and disrupted lightweight Donald Cerrone's leg whips. Diaz won a unanimous decision, 30-27, 30-27 and 29-28.
Diaz (15-7) stood in the face of Cerrone (17-4) in the bout with clear title ramifications, delivering punching combinations to negate the dangerous kicks from the fighter known as "Cowboy."
Although Cerrone dropped Diaz multiple times in the second and third rounds by kicking Diaz's legs out from under him, Diaz persevered with boxing.
Diaz was asked if he thought it was the best fight of his career, and the brother of UFC welterweight title contender Nick Diaz said, "I got to say it was."
Just before that, a vicious knockout came when welterweight Johny Hendricks of Dallas blasted former title challenger Jon Fitch of San Jose with a left hand, pouncing to deliver another blow as referee Steve Mazzagatti rushed in to stop the fight 12 seconds after it started.
Hendricks (12-1) immediately celebrated, but Fitch (26-4-1) was so dazed that he started wrestling Mazzagatti as he revived.
Earlier in the pay-per-view portion of the card, Westminster featherweight Nam Phan (17-11) was overwhelmed by unbeaten Jim Hettes (10-0), who continually pounded Phan's head on the canvas and won a unanimous decision by scores of 30-25, 30-25 and 30-26.
Another Southland fighter suffered a crueler fate when veteran El Segundo light-heavyweight Vladimir Matyushenko (26-6) lost by first-round technical knockout against contender Alexander Gustafsson (13-1).