Sydney Sweeney is a knockout as a coal miner’s boxer

TORONTO — A boxing movie has finally yanked Sydney Sweeney out of her restrictive Hollywood box.

She takes a big swing in “Christy,” the biopic of the “Coal Miner’s Daughter” fighter Christy Martin that had its world premiere Friday at the Toronto International Film Festival.

Instantly, the beaming star of “The White Lotus” and “Anyone But You” disappears.

Well, almost.

“They said it felt like I had demons,” Sweeney’s Martin says at the start in a deep brogue reminiscent of Amanda Seyfried’s Elizabeth Holmes on “The Dropout.”

“Maybe it’s true. Maybe I do.”

Sweeney very well might.

The darkness she tapped into on HBO’s “Euphoria” scarily resurfaces, but in a more mature, extremely challenging role that spans all the way from 1989 to 2012, and includes glorious career highs and unfathomable personal lows. 

Some of those, you’ll wince at.

The quality Sweeney does bring from her fluffier films — and it’s an essential element of her biggest and best performance so far — is an inner light that Martin keeps aflame as her life horrifyingly falls to pieces.

Not all scenes are note perfect, but we’re with her every extremely challenging step of the way thanks to Sweeney’s inviting energy.

For those who don’t remember Martin’s harrowing story, there will be shock and recoiling in the second half.

Even more so because “Christy,” directed by Australian David Michôd with down-under indie brusqueness, starts out as an entertaining underdog sports movie.

When grinning, gung-ho Christy first slugs a man to the ground in the practice ring and quickly yells, “I’m sorry!,” it gets a big laugh.

There’s an in-the-ring montage set to “Bust A Move.”

And her physical change is noticeable.

Sweeney’s Martin has a scissors-gone-wild ‘90s haircut and a “Fee! Fi! Fo! Fum!” gait.

She looks like she can 100% kick your ass and have a grand old time doing it.

It’s funny when Christy adopts a rosy signature wardrobe that’s somewhere between the Power Rangers and the Pink Ladies from “Grease.”

Then a sense of freaky unease creeps in.

It becomes apparent that “Christy” is not really a sports movie at all, at least not as audiences have come to understand them.

Christy isn’t inspiringly trained up like Rocky Balboa — she’s preternaturally talented — and she wins nearly all the bouts she fights. 

That’s just not what this is.

Rather, the film is an often ugly character study of a hard life that only got worse the more famous Martin got.

Born Christy Salters, her domineering West Virginia mother Joyce (Merritt Wever) disapproved of accurate rumors that her daughter was a lesbian.

Not to be deterred, Christy dives headfirst into boxing, and her first trainer James Martin takes disgusting advantage of her and still convinces her to marry him.

Ben Foster, who warps into a villainous sleazeball who looks like he sweats fryer oil, is just as remarkably transformative as Sweeney is.

Eventually, she’s introduced to Don King (Chad L. Coleman, landing punchlines like punches) and goes from a grungy Daytona Beach apartment on the busy highway to a posh house with a hot-pink BMW convertible in the driveway.

“Christy” is about boxing less and less as her marriage, which was always a ruse, corrodes as drugs, money-skimming and abuse become regular events.

She tries to seek refuge in her high-school girlfriend Rosie (Jess Gabor) and her new co-trainer (Katy O’Brian, back in the gloves), but jealous Jim checks her phone and becomes dangerous.  

The final, unsparing half hour will be traumatic for many viewers.

But it could’ve even been more violent and more upsetting. 

Sometimes Michôd discovers the paralyzing horror in not showing every detail. 

And I’m glad the director has found the balance he has.

A hangup I have is that Joyce and Jim are too monstrously evil throughout — almost to the extreme that they could tie her to train tracks in a black-and-white silent film.

There were bad apples, no question about it, but their writing could use more humanity.

Sweeney, though, is a knockout.

And “Christy” is a major step to showing there’s much more to her than rom and com.