Georges St-Pierre, arguably the greatest mixed martial arts fighter of all time, will retire, though lightweight champion Khabib Nurmagomedov has made a last-ditch effort to get St-Pierre to reconsider.

St-Pierre announced his retirement at a news conference in Montreal on Thursday morning.

St-Pierre, 37, had hoped to secure a fight against Nurmagomedov later this year, sources said. But as a deal wasn't materializing, the longtime welterweight champion decided it was time to walk away.

UFC is aware of St-Pierre's decision and is helping to organize the news conference, sources said.

Following the reports of St-Pierre's retirement, Nurmagomedov posted to Instagram saying he would be willing to fight St-Pierre in November and telling GSP, "After this fight you can retire."

"I saw what Khabib wrote, and I appreciate it very much," St-Pierre told ESPN. "Unfortunately, these decisions aren't up to us. I will address everything at the press conference tomorrow."

St-Pierre (26-2), born in Saint-Isidore, Quebec, last fought at UFC 217 in November 2017, when he defeated Michael Bisping to win the middleweight title at Madison Square Garden. At the time, the victory made GSP just the fourth fighter in UFC history to win belts in two weight classes.

Nurmagomedov, undefeated in 27 professional fights, most recently defeated Conor McGregor by fourth-round submission at UFC 229 in December. To challenge Nurmagomedov for the lightweight title, St-Pierre would have drop down to 155 pounds. If victorious, he would become the first to win UFC titles in three weight classes.

St-Pierre fought at middleweight (185 pounds) against Bisping. Nurmagomedov said on Instagram that he would be open to fighting St-Pierre at a catchweight of 160 pounds.

Making his MMA debut in January 2002, St-Pierre started his career a perfect 5-0 before signing with UFC in 2003. He won the welterweight title at UFC 65 in 2006 when he defeated Matt Hughes. He ended up losing the title in his next fight, though, when he was upset by Matt Serra at UFC 69 in 2007. That fight is considered one of the greatest upsets in MMA history. GSP would avenge that loss the following year at UFC 83 and in doing so regained the belt at the first UFC event in his hometown of Montreal.

During his title reign, St-Pierre was viewed as one of the faces of UFC and Canadian MMA. He headlined the promotion's debut in Toronto at UFC 129 in front of 55,724 partisan fans at the Rogers Centre.

St-Pierre walked away from the sport in November 2013 following his split-decision win over Johny Hendricks -- his ninth successful title defense -- though he never officially said he was retiring. St-Pierre's next fight came four years later when he dethroned Bisping at 185 pounds.

Following the fight, GSP stayed in the Octagon a little longer than usual to acknowledge the crowd. That would end up being his final moments as an UFC active fighter.