It ended in heartbreak for both Canadian boxers.
Spencer, a three-time world champion, was ousted in her opening bout last summer in London while Fortin's Olympic dreams ended months earlier when she failed to make the Canadian team.
But when the world champions and longtime friends needed a supportive shoulder, neither could be there for the other. That's because they became fierce foes the day they learned they'd have to fight each other for the one ticket to London, an emotional story that is chronicled in the documentary "Last Woman Standing."
"Honestly it was really hard," Fortin said in a recent phone interview. "I feel like I didn't find anybody else other than Mary, and even though we didn't go through exactly the same thing, I don't feel like anybody else could understand the situation. Nobody but her."
"Last Woman Standing," directed by Juliet Lammers and Lorraine Price, premiers Friday at Toronto's Hot Docs International Film Festival.
Spencer and Fortin were rookies on Canada's national team in 2004, and as Spencer says in the documentary "we bonded right away. Our personalities clicked."
There's humorous footage of early road trips, including one in Argentina where the boxers slept six to a room in a rundown hotel that the owner sprayed daily for bugs.
They dominated their weight classes virtually from Day 1, Spencer at 66 kilograms and Fortin at 70. Spencer would win three world titles while Fortin claimed two.
The announcement that women's boxing would make its Olympic debut in London was bittersweet. Only three of the eight weight classes would be contested, forcing Spencer and Fortin to fight for the one Canadian spot at 75 kilograms.
Two friends became "dead enemies," as Spencer once described their relationship.
"Because we were teammates and friends for such a long time, and we had the understanding and the hope that we would compete together at the Olympics, that's what changed it," Spencer said in an interview. "I'm able to be friends with some people I compete against because we always know we're going to be in competition. But when you have a friend that you share a common dream with, and then one phone call changes all that, that's what made it different."
"We did what we had to do, I would not have changed anything. I feel very happy with the way I managed to No. 1, be a competitor against my friend, but No. 2, to also react to situations the way I did. So I wouldn't change anything, no."
Spencer captured Canada's lone Olympic berth, beating Fortin 18-12 at the national championships in January of 2012. Fortin appealed the result, arguing the bout was fixed. Her appeal was denied, but Spencer said Fortin's claim that the fight wasn't fair was a low blow that took its toll on the fighter from Windsor, Ont.
"Absolutely (it hurt)," Spencer said. "It was something that was a huge distractor for me preparing for the Olympics. I would have loved to have my friend who is an amazing boxer in my weight help me prepare for it. But instead I was seeing stuff in the news about her saying she had won, and this and that, and it was really a distractor."
Their paths went in opposite directions after nationals. Spencer went on to prepare for the Olympics, and in one scene she's shown getting glammed up for a CoverGirl ad.
A forlorn Fortin, shown training alone in her Quebec gym, lost her Own The Podium funding. She launched a bid — that ultimately failed — to box for Lebanon in London.
It was far from smooth sailing for Spencer, who lost her Olympic qualifying bout at the world championships and after several agonizing weeks was granted a wild-card berth. Then Spencer, who was considered a gold-medal favourite in London, was eliminated in her first Olympic bout — a 17-14 loss to China's Jinzi Li.
Spencer said looking back, she believes she may have left everything in that one fight against Fortin.
"That fight that was huge, I put all my concentration into it, I peaked for it, I prepared mentally, physically, emotionally for this fight," Spencer said. "It was over with and then it wasn't like 'Let's go, let's train for the Olympics.' It was having to answer to the media about how the fight went, having to do interviews about 'What's it like for your friend to say she got robbed?' That really was draining.
"Not only did everything go into that fight, but a lot was sucked out of me afterward."
The two 28-year-olds have their sights set on the top of the podium at the 2016 Rio Olympics, and there have been rumblings about increasing the number of weight classes to five.
"That would be amazing," Fortin said.
"That would be great, but I think it's too soon to get our hopes up," Spencer said. "Originally we were told there were going to be five out of 10 (in London), and we made arrangements to fit in those five. And then we got the phone call about three. So it's too soon to get overly excited about that, but that would be fantastic if it came through."
More than eight months have passed since the London Olympics. Have Spencer and Fortin put their fierce rivalry to rest?
The boxers didn't want to spoil the ending.
"Obviously now we're going to keep fighting each other for another four years," Fortin said. "We're not going to do it the same way for sure. Maybe for the first time we needed that barrier because we like each other, we're friends. I don't think we could have stayed friends and kept going like this. We needed some kind of separation, but I don't think it needed to be this deep or intense."