Rory (Ares) MacDonald trains alongside St-Pierre at Montreal's Tristar Gym. Just 23, the native of Kelowna, B.C., is seen by some as the heir apparent to GSP.
Including the 31-year-old St-Pierre himself.
"Oh yeah, definitely. I believe he has all the tools to be the greatest," St-Pierre told The Canadian Press.
Mike Ricci, another Tristar training partner who is currently being showcased on "The Ultimate Fighter" reality TV show, also sees a UFC championship belt being wrapped around his friend's waist.
"Oh yes that's not a question. That's definitely not a question," said Ricci. "He's so talented and so focused, I don't see anybody stopping that man."
MacDonald (13-1) can turn more heads on Dec. 8 when he faces former lightweight and welterweight champion B.J. (The Prodigy) Penn on a televised UFC card in Seattle.
The 33-year-old Penn (16-8-2) is a veteran fighter who loves to get under his opponent's skin. He has used Twitter before this fight to denigrate MacDonald for postponing their meeting due to a cut and to a make some less-than veiled jabs about drugs.
At Penn's request, MacDonald agreed to undergo random drug tests ahead of the fight. MacDonald says he has already passed two random tests, sounding like he agreed to Penn's request just to shut him up.
"At the end of the day, he can say what he wants to say, get people excited but I'm still going to beat his ass," MacDonald told a recent fan question-and-answer session prior to UFC 154 in Montreal.
"He's got a lot of excuses that guy," he added. "He likes to start (stuff)."
Penn is 1-3-1 since the beginning of 2010 years and retired in the cage after his last outing, a lopsided loss to Nick Diaz at UFC 137 in October 2011. That run included losing his 155-pound title and a rematch to Frankie (The Answer) Edgar before beating veteran Matt Hughes, drawing Jon Fitch and losing to Diaz, all three at 170 pounds.
But the mercurial Hawaiian told UFC boss Dana White he wanted to resume fighting, citing MacDonald as a possible opponent. White told MacDonald, who went on a popular MMA radio show to declare his interest in the matchup.
A supremely talented fighter, Penn has been let down by his training regimen at times in the past.
You're never sure what B.J. Penn you're going to get.
"B.J.'s a great fighter," argued MacDonald. "He could show up the best B.J. we've all ever seen the night he fights me. . . . He's going to have to show up, be very impressive to survive what I'm about to put on him that night."
MacDonald, then 20 by a few months, became the youngest fighter on the UFC's books when he signed a four-bout deal in the fall of 2009. Five fights later, he remains the seventh-youngest.
The only person to beat him is former WEC champion Carlos (Natural Born Killer) Condit, who lost to GSP at UFC 154.
After that loss, MacDonald left Kelowna for Montreal to train full time at Tristar. That iron-on-iron training has helped him raise his game each time out since, with wins over Nate Diaz (Nick's younger brother), Mike Pyle and (Beautiful) Che Mills.
He manhandled Diaz, rag-dolling him in the third round en route to a unanimous decision.
MacDonald blunted the veteran Pyle's offence and then took him to the ground and beat him up. It lasted just three minutes 43 seconds with the Canadian holding a 24-3 edge in significant strikes, according to Fightmetric.
Facing a good striker in Mills, MacDonald took him down and then brutalized him on the ground, carving open his face and littering it with lumps.
MacDonald finished off the bloody Brit in the second round. UFC commentator Joe Rogan called it "complete domination by one of the scariest dudes in mixed martial arts."
The Canadian had a 47-6 edge in significant strikes.
MacDonald may be climbing his way up the 170-pound ladder but he and GSP say they will never face off in the cage.
"I don't want to fight him. We fight every day in the gym ... there's a lot of opponents, there are different weight classes," said St-Pierre.
MacDonald is equally dismissive.
"Me and Georges are not going to fight. ... I wouldn't disrespect Georges or the gym in that way."
He wants another shot at Condit with Nick Diaz and Hughes (who appears retired) as options. He has also said he may fight at middleweight at some point.
MacDonald is part of the new breed of fighter. Rather than come to the sport from wrestling, kickboxing or another discipline, MacDonald started training in MMA from the get-go.
He was 14 at the time.
Born in Quesnel, B.C., MacDonald started training with David Lea in Kelowna. He had his first pro fight at 16 in Prince George, because it was the only place to let him fight. Even then his parents had to give their approval.
"It was hard, man," he said of his pro debut. "I didn't know what to expect. I was good in the gym, I trained hard in the gym, but fighting's completely different.
"Having the pressure of people watching you and the lights and stuff, it was different. But I feel pretty confident now where I am. I enjoy the nerves that I get, I enjoy the excitement when I'm in the ring."
He won his UFC debut against Mike Guymon in January 2010 and then was pitted against Condit at UFC 115 in Vancouver in June 2010.
He came out at top gear, taking it to Condit. But the prickly veteran weathered the storm and eventually rallied to stop MacDonald with just seconds remaining.
"People were going insane," MacDonald recalled of the atmosphere at GM Place that night. "I never heard that level of noise in a building ...I was super-shocked and it just got me fired up to a point where it was, like, bad.
"If you watch that fight you could see the intensity that I was bringing and I don't think that was my style. And I paid for it."
He says it taught him "to carry no emotion into fights."
Watch MacDonald fight these days and his face is a blank canvas before the opening bell. His eyes seem vacant.
Listen to how he explains his attitude towards facing Penn.
"I'm willing to die. That (fight) day in my opinion is the last day of my life. That's the way I look at it. He's going to have to show up, be very impressive to survive what I'm about to put on him that night."
MacDonald says there are two sides to him.
"I'm not the same person you're seeing right now when I fight. I'm not a nice person" he said. "I feel like most of the time I'm a pretty nice guy but I have that capability of switching to that other person."
Penn and MacDonald were supposed to fight in September at UFC 152 in Toronto but MacDonald suffered a 38-stitch cut over his right eye in training in August.
Penn ridiculed him for postponing the fight but MacDonald said it just made sense.
"I think me waiting another two months for it to heal properly is a smarter decision for down the road, like years later. Because if this cut re-opened, if I fought him in September, I could have been risking a problem in every other fight down the road. Because it was so deep, it was down to the bone."
The cut came during a regular sparring session on a Saturday. It was his last round and he said he felt great.
He blocked a kick and then hesitated throwing a counter strike, because he was looking at the back of his opponent' head and didn't want to hit him there.
"I should have because I paused and he threw a spinning back elbow and hit me across the eye," he said.
It's the kind of exotic blow that sparring partners usually avoid in training.
"Everyone wants their shot at you, I guess," MacDonald lamented. "Accidents happen, too."
He took two months off sparring, but continued conditioning and other training.
MacDonald is no fan of doing media but says he understands it comes with the job.
He dresses the part of a professional. For the fan session in Montreal, he wore a stylish jacket, tie and dress shirt.
Clothes may not make the man. But they can make him feel good.
"When I was younger I had one set of clothes," he explained. "I had a pair of jeans and a hoodie that I had to wear every day. And I got made fun of for it ... kids tried to beat me up and steal my clothes and stuff.
"I take a lot of pride in being able to afford the clothes I have now and to dress the way that I want. I think it just makes me feel good about myself, maybe."
He hastens to add that he wasn't raised on the streets.
"It just wasn't your suburban-whatever kind of lifestyle, " he said. "It was different ... I don't want to complain about it because, there are many more people that had it worse than me. It was just I had my own struggles just like everybody does. I'm not saying I had it that bad."
While he says he loves his job as a fighter, it wasn't always that way.
Five years ago, he says he was struggling with focus during a training camp. He was distracted and having problem with his girlfriend at the time.
"I was just confused, I was not hungry to fight ... And I just hated being there, hated being in the gym."
His diet was as bad as his attitude.
He won the December 2007 fight — defeating Kajan Johnson for the King of the Cage Canadian lightweight title — then elected to step back from the sport. So he left Kelowna for Langley, B.C., stopped training for five months and started a carpentry job.
"I did that for about five months and I hated it," he said. "And one day I just like snapped, I was just thinking I've got to get back to what I'm supposed to be doing, I'm meant for more than this."
He moved back to Kelowna, and started training again.
He won the King of the Cage world lightweight title next time out, in November 2008. Two wins later, he had a UFC contract. And the road to the title began.
Away from the gym, MacDonald says he is into "regular young guy stuff."
"I like video games, I like shopping, I like girls," he said.