By Paulo Prada

RIO DE JANEIRO (Reuters) - The first day of women's bouts, only the second ever in Olympic boxing, began on Friday with a technical knockout by a Colombian fighter and a victory by Mikaela Mayer, a 26-year-old U.S. national champion making her Games debut.

Mayer, a lightweight, won a unanimous decision against Jennifer Chieng, another U.S. born fighter who trains in New York but was boxing for Micronesia.

A head taller than her opponent and with a considerable advantage in reach, Mayer dominated her bout after Chieng landed a few telling punches early on.

"I might have gotten a little careless," Mayer said about her opponent's initial success in penetrating her guard.

"I do have these long arms. If I am not using them, it's really not worth anything."

Colombia's Ingrit Valencia, a 27-year-old flyweight, beat Judith Mbougnad from the Central African Republic on a technical knockout, repeatedly landing head blows before the referee ended the fight in the third round.

Celebration, however, was not yet on the agenda for Valencia, who is Colombia's first ever Olympian women's boxer.

"I still haven't won anything," she said, noting her desire to win a medal like compatriot Yurberjen Martinez, a flyweight who earlier on Friday clinched a spot in the men's final. "This was just my debut."

In the men's competition, Mongolia's Otgondalai Dorjnyambuu and Sofiane Oumiha of France both advanced to the lightweight semi-finals.

Because of rules that prohibit a third-place fight between those who lose their semi-finals, both boxers are guaranteed at least a bronze medal for their respective countries.

The women's boxing competition, the second-ever following the debut of female boxers in London four years ago, differs from the men's in that the women still wear headgear.

Following a rule change introduced after the 2012 Games, men no longer wear headgear.

Compared with the men's bouts, which feature three rounds of three minutes each, the women's fights consist of four, two-minute rounds.

(Editing by Mark Lamport-Stokes)