The headline event was initially supposed to be a bout between Jiri Prochazka and Glover Teixeira, but Prochazka was forced to vacate his title due to a serious shoulder injury.
After a cautious opening round in which both title contenders took their time sounding each other out, Blachowicz peppered Ankalaev's front leg with kicks and put the Russian under pressure in the second and third rounds.
Ankalaev responded excellently, dragging Blachowicz to the ground in the fourth and fifth rounds, and appeared to have done enough to secure the victory, before the bout was called a draw - just the fifth draw in UFC history.
However, judge Derek Cleary scored the fight 48-46 for Ankalaev, Mike Bell scored it 48-47 for Blachowicz and Sal D'Amato was split down the middle at 47-47, leading to just the fifth draw in UFC history.
"I don't know what to say, I won that fight," Ankalaev, who appeared visibly upset at the decision to call the bout a draw, said with the help of a translator.
"Why didn't I get my belt? I don't know if I'll fight for this organisation again, because honestly, I don't know what just happened. I don't know what to say, maybe we'll come back and take what is ours."
In an act of sportsmanship, Blachowicz, who previously held the belt between September 2020 and October 2021, said the win should have been awarded to Ankalaev and tried to raise his opponent's hand after the fight.
"I have to watch the fight again, but for sure, I did not win the fight. I don't know if I lost the fight, but for sure I'm not the winner," Blachowicz added.
In the co-main event, Paddy "The Baddy" Pimblett beat Jared Gordon in a lightweight bout, but the judges' unanimous decision in favour of the Englishman failed to adequately tell the story of a bout in which he was pushed to the limit by his American opponent.
"Jared is tough, he's a tough man," Pimblett said. "He's come back, hats off to him. I knew I won the first and second so coasted in the third not wanting to take any stupid risks. I'm not going to lie, I hurt my right foot in the first round."
On the undercard, 18-year-old Raul Rosas Jr became the youngest fighter to win a UFC bout, beating Jay Perrin via submission on his debut
