Claressa Shields took out her medal from 2012 during her award ceremony Sunday. The middleweight from Flint, Mich., became the first U.S. boxer to win two golds at the Olympics. (Valdrin Xhemaj/European Pressphoto Agency)

When she made history by winning gold in the 2012 Olympic debut of women’s boxing, Claressa Shields, a 17-year-old high school junior from Flint, Mich., was a portrait of stoicism on the medal stand.

The jab wasn’t the only thing she had mastered growing up with one parent in jail and the other unable to care for her. She had become an expert at keeping the pain and scant joy that defined her childhood tucked deep inside, as if trained to place gloved mitts over her soul, too, just to make sure no one got inside.

At the Rio de Janeiro Olympics on Sunday, after she defended her 2012 gold by unanimous decision, dismantling an imposing Dutch opponent with flailing arms, Shields let her emotions fly.

She raised her arms in victory even before the referee did so officially. Then she hopped down from the ring, grabbed a giant U.S. flag and circled the arena, flag held overhead, as if she were about to take flight. And after the gold medal was placed around her neck on the podium, Shields slipped her 2012 gold medal with the purple ribbon from her USA jacket pocket and put that on, as well.

She had planned this months ago. Now 21 and a two-time Olympic champion, Shields wanted to make sure the world knew she had done something historic — becoming the first U.S. boxer, man or woman, to win two Olympic gold medals. She wanted to scream it to the heavens. Instead, she settled for a huge smile, fingering the medals that said more than words as the United States’ national anthem played for the first time at Rio’s boxing venue.

“After I won a gold medal [in 2012] and I got home, my gold medal didn’t really mean a lot to the media and, I guess, to the boxing world either,” Shields explained afterward. “But I have a great story to tell. I’ve been through a lot.”

Shields’s triumph over middleweight Nouchka Fontijn of the Netherlands gave USA Boxing its only gold of the Rio Olympics. Bantamweight Shakur Stevenson, the 19-year-old from Newark whom Shields regards as a younger brother, claimed silver. And flyweight Nico Hernandez of Wichita took bronze.

It was Shields’s third unanimous decision in as many fights. In her two previous fights, she had coaxed a standing eight count in the fourth and final round. In recognition of her dominance, Shields was awarded the Val Barker Trophy as the most outstanding boxer at the 2016 Olympic Games.

Stevenson’s loss by split decision in Saturday’s gold medal bout shook Shields. It didn’t diminish her love or respect for her U.S. teammate, but it reminded her that even the best boxers don’t win every fight.

So she told U.S. Olympic boxing coaches Billy Walsh and Kay Koroma that she wanted to be told after every round whether she had won it. They needed to push her — to make sure she stayed the aggressor. There could be no doubt in judges’ minds who was the better fighter.

The first round was tough, with the 5-foot-11 Fontijn keeping Shields at distance with her long arms. Shields waited for an opening while frustrating Fontijn with deft evasive moves. And when the 5-9 Shields connected, the whole arena felt it. Smack, as Fontijn’s head snapped back. By the fourth round, Shields was flapping her arms at her opponent, begging her to step it up.

“Fantastic performance!” Walsh gushed. “There was a lot of pressure on her. But as the fight went on, we just told her, “Enjoy this! Show the people what you can do!’ ”

Boxing saved Shields from the moment she took her first lesson at age 11. It was an outlet for her anger. It gave her structure. It gave her purpose. And as her punches got heavier and her footwork quicker, it convinced her that she could go as far as hard work would take her.

On the final day of the 2016 Rio Olympics, boxing took Shields further still, bringing her record to 77-1 and adding “two-time Olympic gold medalist” to a résumé that already included “two-time world champion.”

And though she had said all along that there was no “if” in her pursuit of a second Olympic gold — victory was certain, she vowed — all she could do in the immediate aftermath was repeat over and over, “I can’t believe it! I’m so happy!”

But it will mean little, in Shields’s mind, if no one notices; if corporate sponsors don’t recognize her marketing power; if boxing promoters don’t see the entertainment potential of powerful, skillful female fighters; if African American women don’t see the beauty of their skin complexion and hair texture; or if children with absent parents in rough cities like Flint don’t believe they can make their way in life, too.

“I want to inspire people and give people just a little bit of hope,” Shields said. “I remember when I was one of those little kids who didn’t have any hope. And when I got just a little bit of hope, look how far I’ve come. That’s what I want to do for those kids.”