Sweeney, who has made negative headlines recently for controversial jeans ads, has become a contender for the Academy Awards for her portrayal of pioneering female fighter Christy Martin.

The movie had its world premiere this weekend at the Toronto International Film Festival, where Sweeney's performance and physical transformation drew glowing reviews.

Though ostensibly a boxing biopic, "Christy" swiftly develops into a surprisingly dark tale of domestic abuse and misogynistic control, as Sweeney's pugilist falls under the jealous and violent sway of her coach and husband Jim.

As well as visceral and bloody battles in the boxing ring, the film calls on Sweeney to take on many daring and deeply unglamorous scenes.

"Sydney's up for anything -- whatever the movie needs, she'll go there," director David Michod told AFP.

In one scene, her character is coerced by her husband into shooting a disturbing homemade sex video involving a fake penis.

In another, he essentially pimps her out to a man who pays for a highly sexualized grapple in a motel room, which makes for difficult viewing.

In real life, a broke Jim really did "set up these scenarios where men would pay to spar with her," explained Michod.

"She wasn't having sex with these guys... But it's a kind of weird sex work."

'Creature of the internet' 

It is all a far cry from Sweeney's glamorous public persona.

Having broken through with roles in acclaimed HBO series "Euphoria" and "The White Lotus," the in-demand actor had become the lucrative face of several brands.

Most notably, a series of American Eagle clothing ads proclaiming the blue-eyed and blonde-haired star "has great jeans" were accused of evoking eugenics and even having white supremacist undertones.

Sweeney swiftly shut down any questions on the topic at the Toronto premiere of "Christy."

But Michod said the controversy had never fazed his star.

"Sydney is a creature of the internet," said the director. 

"She has a way of just moving through this stuff. I mean, yeah, she's healthily ambitious, and she's very good at what she does. 

"She's out there in the world. And I'm actually very admiring of her, and I've stopped worrying about things that I can't control."

'Playful' 

The brouhaha does not appear likely to impede Sweeney's nascent Oscar bid. The film is set for release in November -- a prime slot for awards campaigns.

And reviews for "Christy," though mixed overall, were nearly unanimous in their praise of the 27-year-old's performance.

The real Christy Martin, the daughter of a coal miner, became boxing's first true female star, but for decades had to suppress her sexuality and was abused by her trainer and eventual husband Jim Martin.

Martin stabbed and shot his then-wife and remains in prison over her attempted murder.

Tales of Sweeney's fight training and her diet of Chick-fil-A sandwiches and milkshakes to gain weight have already been trotted out at the premiere, and will be a staple of the upcoming awards season.

Perhaps more impressively, Sweeney is called upon to play Martin across more than 20 years of her life, from a vulnerable and naive teenage ingenue to a grizzled abuse survivor.

"She'll do anything. She's just fearless," said Michod.

Sweeney brought a "playful" energy to set every day, regardless of the scene being shot, he said.

Even that creepy sex tape scene?

"I mean, it's a fake dick," laughed Michod.

"You're going to find humor in it, however inappropriate."