"It showed at the gym. I wouldn't have enough energy," she said. "I performed bad and I'd get super tired. I wouldn't be able to recover. I was in great shape, but you could just tell I wasn't eating."
While her younger siblings, Brianna Shields and Dusable Lewis, leaned on her, Shields learned early on that the only person she could rely on was herself. Even if that meant walking alone in the cold and dark so she could be at the gym by 6 a.m. to get a ride to a tournament.
"I didn't want to have to depend on nobody," she said. "So I would get up and walk."
Shields gravitated to boxing because of her father, whose own career as a fighter was derailed by his troubles with the law. He would tell her about Muhammad Ali and how his daughter, Laila, followed in his footsteps.
Shields decided she would do the same.
"Normally when kids have personal issues, they respond negatively," said Cheryl Adkins, principal at Northwestern High School, where Shields is a senior. "Claressa decided this was going to be her road and she stuck to it."
Shields was 11 when her father signed the permission slip for the boxing program at Flint's renowned Berston Field House. Crutchfield told her to join the other kids in front of the mirrored wall in the cramped basement gym to practice technique, then handed her off to one of the other coaches.
He didn't bother learning her name. He doubted she'd last long enough.
Two weeks in, however, Crutchfield noticed that Shields was catching on faster than the boys. And while she didn't say much, there was a fierce intensity to her.
"I saw how good she was doing and how fast she was advancing and then that's when I grabbed her," Crutchfield said. "I said, 'What's your name again?' She said, 'Claressa.' I said, 'Nah, from now on your name is Ress.'"
Crutchfield took Shields under his wing, teaching her the same punches, footwork and strategies that had won him four Golden Gloves titles in Michigan in the 1980s. She was a natural, and when women's boxing was added to the Olympic program in 2009, Crutchfield told Shields she could win the gold medal.
She was 14, not even old enough to qualify for the U.S. championships.
"I ain't never seen a woman who boxes like me. Even the girls who won gold medals," Shields said, proudly. "I think if I was another girl and I had to fight myself, I'd be biting my fingernails."
There is a brutal elegance to Shields' fighting style. Her fists fly with a smoothness, and she delivers her punches with a rhythmic POP! POP! POP! But the blows are punishing and come with unrelenting force, a power fueled partly by rage.
Shields talks matter-of-factly about her family and upbringing and the challenges they presented. She has made peace with all parts of her story, recognizing that while others may have provided the material, it is up to her to decide how it is written.
"You really can't do nothing about stuff you can't control," she said. "You can't control other people."
But that wasn't so easy to understand when she was younger, and the upheaval could be overwhelming. She would lash out and throw tantrums, and Crutchfield would kick her out of the gym as punishment.
"Boxing has helped me control my anger outside the gym," Shields said. "So now, whenever I experience something outside, I don't even let it affect me no more. It's stuff that used to make me just snap."
Like her family, and having to look after herself much earlier than any child should.
"Sometimes I think about other people's families. ... They're all, like, super close," she said. "My family used to be like that, but they kind of broke up and now they're not. Sometimes I think about that, but I really can't do nothing about it. As long as I've got somebody, I'm all right."
That somebody is Crutchfield and his wife. Or "Mama Mickey," as Shields calls her.
Crutchfield knew Shields didn't have a perfect home life. Few of the kids at Berston did. But when he realized she was hungry and walking by herself to the gym, he knew he had to step in.