By Alan Baldwin

RIO DE JANEIRO (Reuters) - Reigning Olympic champion Claressa Shields of the United States made sure of another medal after reaching the women's middleweight semi-finals on Wednesday.

The 21-year-old beat Russian Yaroslava Yakushina on a unanimous points decision in her first fight of the Games after receiving a bye through the opening round.

Kazakhstan's Dariga Shakimova, who beat Morocco's Khadija Mardi in her quarter-final, now stands between her and another final.

"I would give myself an A+ if I was to look at how I reacted to my body," Shields told reporters after a fight that she never looked in danger of losing. "My body wasn't responding the way I wanted it to.

"I hit her with a lot of good jabs, a lot of good right hands...I did a good job transitioning but as far as the overall performance, about a C."

Shields has not lost since she won gold in London four years ago to become the first U.S. woman to win an Olympic boxing title. She could now become the first U.S. boxer since 1904 to win two golds.

Since London she has won two world championships and the Pan American Games.

While she said it was good to end the waiting, and she had not fought since the world championships at the end of May, she brushed aside any sense of achievement in winning a medal at two successive Games.

"Just bronze or silver, that's not my thing," she said. "I want to be a two-time Olympic gold medalist and I think I'm getting pretty close."

Shields said she had not been too concerned by a controversy over the scoring either.

"The judges can't beat God," she said. "I just prayed that he be with the judges and let the judges be fair and give the fight to the person who earned it."

The other middleweight semi-final will be between China's Li Qian, who beat Brazilian Andreia Bandeira, and Nouchka Fontijn of the Netherlands, who out pointed Britain's Savannah Marshall 2-0 to secure her country's first boxing medal since 1992.

The women's lightweight final will be between China's Yin Junhua, who out pointed Finland's Mira Potkonen, and France's world champion Estelle Mossely who beat Russian Anastasia Belyakova on a technical knockout.

Potkova and Belyakova both get bronze medals.

(Reporting by Alan Baldwin; editing by Andrew Hay)