LAS VEGAS -- On a night when Brock Lesnar was at his most vulnerable, he pulled out a career-best effort.
The heavyweight champion retained his title because he refused to lose it Saturday night at UFC 116.
Interim titleholder Shane Carwin dominated Lesnar in the first round. He got Lesnar on the ground for several minutes and beat him up good.
It was a situation Lesnar never experienced in the Octagon. He was dazed, bloodied and on the verge of losing his crown.
Carwin, who finished 12 previous opponents when he had them hurt, appeared to have this fight in the bag. There was no reason to believe that Lesnar would avoid becoming the 13th casualty.
But he did.
Lesnar survived and showed that he is a special champion. He will not be dethroned easily.
"Brock Lesnar has been dominating everybody since he came into this sport," UFC president Dana White told ESPN.com. "Tonight he absolutely got dominated in that first round by Shane Carwin.
"You find out how tough a fighter is when they face adversity, when somebody is on top of you, dropping punches down on you that have knocked a lot of other people out. Brock stayed in there, came back in the second round and pulled off a submission."
If this fight had taken place in November 2009, as originally planned, Lesnar would not have survived the first-round pounding Carwin put on him. That version of Lesnar was a very sick man who would eventually be diagnosed with mononucleosis and later diverticulitis.
According to doctors, Lesnar fought '09 at about 60 percent. He entered this fight against Carwin at 100 percent and needed every ounce of his strength to retain the belt.
That Lesnar needed to be at his physical best against Carwin wasn't a secret. Seconds into the fight, however, it was clear that Lesnar needed something more.
He had to dig deep inside himself and find that extra willingness to survive.
It wasn't an unusual request. For the past year, Lesnar's willingness and ability to overcome a major hurdle has been called upon numerous times.
Lying in a hospital bed last year while unable to eat solid foods and not knowing whether his fighting career was over, Lesnar refused to quit. He dug deep within and survived.
He summoned his body to do the same thing Saturday at MGM Grand Garden Arena, and it responded.
"It's been a roller-coaster ride all over the place," Lesnar said. "The thought did cross my mind when I was underneath [Carwin]. I've been through a lot, and to weather that first round, I thought to myself, 'This isn't how it's going to end. I've got to keep moving down here.'
"He outboxed me. He got me, and I will be the first to tell everybody that. That's just the way it is.
"It's been a crazy year. I've been through a lot. I wasn't going home without this [belt] tonight. That's for sure."
Lesnar (5-1-0) keeps his belt, and it will be more difficult for the next man -- Cain Velasquez -- to take it away from him. Just as he did with the illnesses he battled last year, Lesnar came face to face with his vulnerability in the cage.
Carwin proved to the world that Lesnar can be beaten. If Carwin gets a second shot at Lesnar within the next 18 months -- which isn't out of the realm of possibility -- he will likely pace himself better.
When Carwin had Lesnar hurt, he became overanxious and punched himself out. Even Lesnar acknowledged that each shot from Carwin did less damage than the one before.
Carwin (12-1-0) was breathing heavily at the end of the first round. And though he winked to the crowd and wore a huge smile before the start of Round 2, Carwin had not fully recovered.
It is unlikely he will punch himself out again, but getting Lesnar in the same situation will prove difficult. Lesnar too will be a better fighter the next time out.
"Was I better tonight than I was a year ago? I would hope to think I am," Lesnar said. "Each win you have to put behind you and move forward.
"We'll train and keep doing the right things. I'm a wrestler; I'm trying to evolve.
"Is it remarkable that I only have six fights and I'm UFC champion? Of course it is. But I've got a lot of room to grow."
Franklin McNeil is a contributing mixed martial arts/boxing writer for ESPN.com. He also appears regularly on "MMA Live," which now airs on ESPN2.