UFC boss Dana White wanted to pit St-Pierre against Anderson Silva, a bout many fans have waited for, but the champion from Montreal asked for Diaz.
"Georges has been a great champion and he's been great for the company and if he wants another fight, we'll make it for him," White said Wednesday at a news conference attended by a throng of media and about 100 fans. "We knew Nick wanted the fight, so it worked out."
It will be the main event on a card that features six of the top-10 ranked welterweights.
The co-feature between St-Pierre's training partner Rory (Ares) MacDonald (14-1) and Carlos Condit (28-6) and another 170-pound bout pitting Johny Hendricks (14-1) against Jake Ellenberger (28-6) may help decide who St-Pierre's future opponents will be, if he stretches his winning streak to 11 by defeating Diaz.
"This is the big fight that people wanted to see, more than any other guy," said St-Pierre (23-2). "For me, Nick is the No. 1 contender.
"There's a history between us. We both wanted this fight for a long time. Let's do it."
The two were scheduled to square off nearly two years ago, but Diaz (27-8-1) was dropped from the bout when he failed to turn up at two promotional events.
St-Pierre mentioned he was glad to see Diaz present and accounted for.
"I'm just happy to get the show on the road," said Diaz, the former Strikeforce champion. "I didn't make it to the last press conference.
"It wasn't very professional on my part. I wanted to fight, but I had to learn my lesson."
The 31-year-old St-Pierre returned from an 18-month injury layoff on Nov. 17 to defeat Condit by unanimous decision and retain the championship belt.
Another lesson for Diaz came last May when he was suspended for a year and fined a total of US$79,500 by the Nevada State Athletic Commission for testing positive for marijuana metabolites, retroactive to his Feb. 4 loss to Condit for the interim welterweight title.
That put the 29-year-old on the longest leave from the octagon he can remember. But he has been training, feels good because old wounds have healed, and said he's ready.
"I didn't think I'd get this fight," said Diaz, who fights out of Stockton, Calif. "I thought I'd have to fight my way back to the big fights.
"(St-Pierre is) the best. He's got the spot. He's the guy to beat."
Despite his victory, St-Pierre emerged from his comeback bout with his face a bloody, swollen mess, and Diaz made further controversy by tweeting that he wasn't impressed.
But Diaz said the tweet wasn't meant to criticize the champion, and St-Pierre let it pass, saying he doesn't pay attention to what's said or written about him.
"I'm not a coward, I never run away from anyone," said St-Pierre. "He deserves his chance and now I'm going to give it to him.
"I asked for Diaz for different reasons. It's a fight that should have taken place a long time ago but didn't happen. We still have things to settle."
St-Pierre will have a second straight fight in his hometown.
Attendance at the usually sold-out Bell Centre dropped to 17,000 for St-Pierre's last fight, but White said he is not concerned because it still brought in $3.1 million at the gate.
"The day I start bumming out over $3.1 million it will be a very weird day," he said.
Other bouts have Patrick (The Predator) Cote (19-8) of Montreal facing ex-Strikeforce fighter (Vicious) Bobby Voelker (24-8) in his UFC debut.
Middleweight Nick (The Promise) Ring (13-1) of Calgary faces Chris Camozzi (18-5).
Jordan Mein of Lethbridge, Alta., and Sean (The Punisher) Pierson of Toronto are also on the card along with Montreal fighters John Makdessi and Yves Jabouin.
Tickets for the general public go on sale Saturday.
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