The actress has been tapped to play brawler Christy Martin in a new biopic -- and yes, she's gonna have to get in fighting shape for the lead role ... which will likely see her bloodied and bruised, the way CM used to be in her heyday when she would brawl in the ring.

Sydney is ready to take on the challenge, as she shared with Deadline that she's prepared to tap into her sporty roots for the role.

She said ... "I grappled and did kickboxing from 12-19 years old. I’ve been itching to get back into the ring, train, and transform my body. Christy’s story isn’t a light one, it’s physically and emotionally demanding, there’s a lot of weight to carry. But I love challenging myself."

Perhaps this new tough role will put Sydney's naysayers to rest. Remember, Sydney was put on blast by Hollywood producer Carol Baum, who said she couldn't understand Sydney's appeal as she felt the young star wasn't talented or all that attractive.

Many of Sydney's famous friends came to her defense ... applauding her looks and her natural talent.

Sydney is known for being quite the bombshell ... putting her curves on display in the rom-com "Anyone But You," as well as HBO shows "Euphoria" and "The White Lotus."

Yet, she's taken a more serious turn recently ... just take a look at her work in "Immaculate," in which she played an American nun uncovering a dark secret at an Italian covenant.