Girl in the Red Corner, by Stephen Spotswood

Unemployed, fresh off a dead-end marriage, and searching for a way to take control of her life, Halo walks into a gym and signs up for mixed martial arts lessons. Her trainer thinks she’s a light-weight. Her mother and sister think it’s just plain weird.

Very quickly the lessons she learns in the ring bleed into the rest of her life and Halo finds herself battling against everyone around her. As she prepares for her first match, Halo realizes that life inside the cage and outside the cage are disturbingly similar. And that the only way to survive either is to fight.