By Alan Baldwin

RIO DE JANEIRO (Reuters) - Claressa Shields brought one gold medal to the ring with her and left with two hanging around her neck after retaining her Olympic middleweight title and making U.S. boxing history on Sunday.

As a gesture of confidence, having the 2012 gold in her pocket before the fight in the expectation of showing it off alongside another took some beating.

But so too does Shields, the 'T-Rex' who has not lost since 2012 and is her country's first double boxing champion in 112 years.

Her two golds are two more than any other U.S. boxer has won since 2004.

"At London I knew I was going to win and I knew I was going to win here," she told reporters after the unanimous decision over Nouchka Fontijn of the Netherlands.

"I've worked so hard to be here. You know not everybody can be an Olympic gold medalist. I'm a two-time Olympic gold medalist. Oh my god, I can't believe I just said that," she exclaimed.

Shields knew long before the result was confirmed, and the winner's hand raised by the Vietnamese referee, that the gold was hers.

Climbing out of the ring after the announcement, she ran around the arena with the U.S. flag held aloft before wrapping it around her.

"She convinced herself from the moment she got here that she was going to win that second gold medal and had it (the 2012 one) in the pocket waiting," U.S. coach Billy Walsh told Reuters.

Officials later declared Shields the co-winner, a first in Olympic boxing, of the Val Barker prize awarded to the best fighter of the tournament. She shared the honor with Uzbekistan's light-flyweight Hasanboy Dusmatov.

TWO TOGETHER

Joined on the podium by bronze medalists Dariga Shakimova of Kazakhstan and China's Li Qian, the fighter from Flint, Michigan, slipped the gold won in London four years ago around her neck to stand and sing the anthem.

Shields had won the first three rounds 10-9 10-9 10-9 but the fourth was closer, with two of the three judges giving it to her Dutch opponent even though the American felt she had been in complete control.

"The last round was like: 'Hey, you want to fight, you think you can beat me, OK let's go'. She's powerful and I'm like, 'What you going to do about that?'," said the champion.

The fighter, whose father spent the troubled first nine years of her life in prison for robbery, now has a 76-1 amateur record and she hoped her example would inspire more black women to overcome adversity and excel.

"There have been times when I wished my hair wasn’t so thick, but at the same time I love everything about me. Black women doing great things is magic," she said.

The last American boxer to win two gold medals was Oliver Kirk, who claimed bantamweight and featherweight titles at the same 1904 St. Louis Olympics, where only U.S. boxers took part.

Some historians have questioned that tournament's Olympic status.

Women's boxing was introduced to the Olympics in 2012 and Shields follows Britain's Nicola Adams, who successfully defended her flyweight title on Saturday, as a double champion.

(Additional reporting by Angus MacSwan; Editing by Bill Rigby and Ken Ferris)