Mother-of-two Rachael Buckland, 25, fractured her skull and suffered a bleed on the brain after the fall at her home in St Mellons, Cardiff.
She was taken to the city's University Hospital of Wales where she has undergone brain surgery.
Her husband Mitchell Buckland, 23, also a professional fighter, said she was "fighting for her life".
The couple met while training together at St Joseph's boxing gym in Newport.
Mrs Buckland, a chef, is a former Welsh bantamweight champion but took time away from the sport after giving birth to children Lorenzo, two, and Ella-Rose, one.
"It's just a waiting game now so still praying but she's stable," Mr Buckland said.
Tony Borg, boxing coach at St Joseph's, said: "It's affected everybody at the gym. We're trying to keep focus for Mitch; he rings a few times a day with any updates - we want to keep positive for him.
"We're trying to do what we can. They are a young couple with two young children. What they are going through now is unreal."
A former boxing champion is in a coma after falling down a flight of stairs.