Mary Spencer was a highly successful amateur before switching to the professional game at 36 in 2021.

Since then, she's been on something of a mission. However, a pair of 10-round defeats to Femke Hermans - one unanimous, the other majority decisions - 11 months apart threatened to derail those plans.

But like all good champions, she dusted herself down and found a way, picking up the interim WBA junior middleweight title with a points win over Naomi Mannes last September and subsequently upgraded to full titleholder earlier this year.

The now 40-year-old enjoyed a homecoming defence and is now ready to face one of the biggest names in women's boxing as Mikaela Mayer again steps up in weight, headlining Thursday's Montreal Casino show.

The fight has been given a late boost when news filtered through Tuesday that the vacant WBC and WBO world titles will also be available for the winner, making it a unification.

Spencer-Mayer and select undercard bouts will stream on punchinggrace.com and Top Rank Classics on Top Rank Classics FAST channel beginning at 8 p.m. ET/5 p.m. PT.

"I think it's a dream come true," Spencer (10-2, 6 KOs) told The Ring. "Every time I would watch her fighting previously, I even said, 'I wish I fought someone like her.' I didn't think in a million years, I would fight her, seeing when she was 135, but I am thrilled to be going against her now.

"It really feels like a huge gift, not only am I fighting Mikaela but I'm fighting her here at home. I couldn't believe it when I was offered this fight, it's very exciting."

Spencer, The Ring's No. 4-rated junior middleweight, is known for her aggression and power but earlier this year, had to display underrated skills when outboxing Ogleidis Suarez on April 10.

"I had to box well, I had a broken right hand for 10 rounds," she confirmed.

"I went in with a broken right hand. Early in the fight, a headbutt broke my nose. I passed a lot of tests in that fight and most people didn't realise, not only did I box without getting tired - which I usually do - I boxed and moved behind my jab out of necessity, weathered a broken nose with blood dripping down my face without being deterred from the plan."

"For me, a lot of people might look at it as, 10 rounds with Suarez but I know I checked a lot of boxes for that fight. I feel confident about my next one, having had that one previously and I'm really looking forward to this."

Spencer had surgery on the broken hand in the days post-fight and feels in a good place.

"I had enough time for it to heal, rehabilitate and I'm excited to use it in this fight," she continued. "It was broken and displaced in my last fight, but I still threw it. Even if it was still a little funky, I'm using my right hand."

Facing someone of Mayer's stature will afford Spencer the opportunity to make a statement.

"It certainly will because a lot of people they do just look at the visibility of a fighter, or the accolades you can put behind a name their name, two-weight world champion - I don't look at it that way," she explained. "I look at their skill set, who they beat. If people want to look at Mikaela as a huge star and test for me, that's great, I can't wait to go and pass that test.

Spencer is dialed in and ready for a fun shootout.

"I think her No. 1 asset is her heart and guts," she said. "I like what she's doing for woman's boxing, fighting anybody, bringing the fights, making it exciting for the fans, even has a little bit of trash talk and I think it's not going to be helpful if she fights me that way, but I appreciate she does.

"She puts punches together, has all the punches you can think of but I'm not impressed by certain things I think are going to make a difference. She's a smaller person moving up, if she had more athleticism and speed, that might be something to think about, but I think she boxes like a big girl, but she's not one - it's going to be a fun fight."

Mayer, The Ring's No. 1-rated welterweight contender, was similarly a standout in the unpaid ranks and represented America at the 2016 Rio Olympics.

She worked her way up before winning the vacant WBO junior lightweight title with a 10-round points win over Ewa Brodnicka in October 2020. The 35-year-old added Ring and IBF titles when outpointing Maiva Hamadouche in November 2021.

Mayer (21-2, 5 KOs) narrowly came up short against WBC titleholder Alycia Baumgardner, losing a split decision in a further title unification.

Unperturbed, she bounced back with two victories before being edged out in another split decision defeat, this time against Natasha Jonas for the IBF welterweight title.

Since then, Mayer has dethroned and subsequently defended the WBO welterweight strap during an entertaining two-fight series with Sandy Ryan, concluding in late March.