Khabib Nurmagomedov kneeled down and put his head in his hands. Tears streamed from his eyes. The burden he had been carrying for the past three months had been released. The typically stoic champion's emotions got the better of him.

Nurmagomedov, one of the best fighters in mixed martial arts history, submitted Justin Gaethje -- whom many said would be Nurmagomedov's toughest test -- via triangle choke at 1:34 of the second round Saturday in the main event of UFC 254 in Abu Dhabi.

Nurmagomedov took Gaethje down, applied the triangle choke, cinched it in and Gaethje went completely unconscious. Nurmagomedov's triangle-choke submission finish was just the third-ever in UFC title history, according to ESPN Stats & Information.

The victory unified the UFC lightweight title.

This was Nurmagomedov's first fight since his father and head coach Abdulmanap died in July due to complications from COVID-19. And afterward, Nurmagomedov announced his retirement from mixed martial arts.

Nurmagomedov, 32, said his mother did not want him to compete without his father, but he told her this would be his last fight.

"I promised her this was gonna be my last fight," Nurmagomedov said. "If I give my word, I have to follow this."

Nurmagomedov said he had no interest in fighting the winner of a proposed January bout between Conor McGregor and Dustin Poirier. Nurmagomedov said he has already choked both men out. He left his gloves in the center of the Octagon before departing.

"Today, I want to say this was my last fight," Nurmagomedov said. "... It was my father's dream. What else?"

ESPN has Nurmagomedov ranked as the top pound-for-pound MMA fighter in the world, with Gaethje at No. 8. In the lightweight division, ESPN has Nurmagomedov and Gaethje at Nos. 1 and 2, respectively. Nurmagomedov came in as the UFC lightweight champion and Gaethje was the interim champ.

"I know he made his father so proud," Gaethje said.

After the fight, Nurmagomedov thanked the UFC and several top executives, but had one request. "I know only one thing I want from UFC: You guys have to [rank me as the] No. 1 pound-for-pound fighter in the world, because I deserve this."

The bout took place without fans at the Flash Forum on Yas Island, which the UFC has dubbed "Fight Island," the promotion's international home during the pandemic.

Gaethje won the first round on the cards of judges Sal D'Amato and Ben Cartlidge. He landed hard combinations in the round, although Nurmagomedov kept the pressure on, had a good jab and landed a takedown at the end of the round. The round finished with Nurmagomedov working for an armbar submission. Judge Derek Cleary scored the first for Nurmagomedov.

Nurmagomedov (29-0) came in undefeated, and this victory ties him with BJ Penn and Benson Henderson with the most title defenses in UFC lightweight history (3). The native of Dagestan in Russia owns victories over the likes of former champions McGregor, Poirier and Rafael dos Anjos. Nurmagomedov is a perfect 13-0 in the UFC, only the second fighter after Anderson Silva to start a UFC career with 13 straight wins.

Gaethje (22-3) was on a four-fight winning streak coming in and beat Tony Ferguson to win the UFC interim lightweight title at UFC 249 in May. The Arizona native had not been to a decision in six years, with nine KO/TKO victories in that time. Gaethje, 31, is the former World Series of Fighting lightweight champion.

"I felt alive," Gaethje said. "That's why I step in here, man. I ask my coaches if they're ready to feel alive every time I step in here."