Claressa Shields, a teen boxer from Flint, Mich., won her bout in the Olympic Boxing semifinals on Wednesday, positioning herself for a run at the gold medal during the finals tomorrow.

The 17-year old from Flint, Mich., not only beat Kazakhstan fighter Marina Volnova, she nearly doubled her opponent’s points. Ahead every round, Shields’s final score was 29 to Volnova’s 15.

“Once again, I refuse to lose,” she said, celebrating backstage with her coaches and some of the best-known women boxers of the past.

(Listen to our special following Claressa and the women boxers who competed to go to London, Go For It: Life Lessons From Girl Boxers)

The victory Wednesday means T-Rex, as she's known by fans, is guaranteed a gold or silver medal when she competes against Russian middleweight Nadezda Torlopova.

Shields stepped out of the ring with a radiant smile, her fans shouting ecstatically.

Longtime boxing blogger Malissa Smith of Girlboxing tweeted, “Tears in my eyes, this 58 yr old girl boxer with a bum shoulder is awestruck and inspired.”

In a nail-bitingly close bout, Marlen Esparza, the Cover Girl-sponsored flyweight from Houston, fought courageously against favorite Ren Cancan of China but she couldn’t close the gap. Esparza lost the fight by two points. Her score 8 to China’s 10.

Esparza has big plans for her life after the Olympics. At a tournament in Canada she told Women Box she doesn’t want to become a professional boxer. She wants to go to medical school and become an anesthesiologist. Esparza was a high school honors student and put off college to box.

Sue Jaye Johnson contributed reporting