Liverpool lightweight Natasha Jonas became the first female Briton to win an Olympic boxing match as the women's event made its debut at the Games.
American Quanitta Underwood came out swinging and led after round one, but Jonas found her range in the second, landing with some sweet right hooks.
Underwood took a standing count in the fourth as Jonas made her superior boxing skills tell to win 21-13.
Jonas fights four-time world champion Katie Taylor for a medal on Monday.
Jonas, 28, became the first British female boxer to qualify for an Olympics by winning bronze at the World Championships in China in May.
She went into her fight against Underwood, 28, as favourite, having beaten the former World Championships bronze medallist at the Olympic test event in November 2011.
Jonas faces a monumental task to win a medal, as Taylor, a legend in her homeland of Ireland, has won five European as well as four world titles.
She said of Taylor: "She is a fast puncher, she can box, she can fight. But I gave her too much respect the first time I boxed her. But that was two years ago. If I didn't think I could beat Taylor, I wouldn't be here."
British flyweight Nicola Adams and middleweight world champion Savannah Marshall received first-round byes and also box for medals on Monday.
Adams's first fight is against Bulgaria's Stoyka Petrova, while Marshall faces either Marina Volnova of Kazakhstan or Elizabeth Andiego of Kenya.
Earlier, Russian flyweight Elena Savelyeva won the first-ever women's boxing match at an Olympics, beating North Korea's Hye Song Kim 12-9.
Savelyeva said it was "a pleasure to make history" after setting up a medal match against world number one Ren Cancan of China.
"I tried to show my pride, it was an amazing thing to do," said Savelyeva, who became a boxer after a trainer at her self-defence class recommended she give it a try.
"It's a normal thing in Russia. There is no discrimination against female boxers. There was no pressure."
India's five-time world champion Mary Kom was close to tears after beating Karolina Michalczuk of Poland 19-14.
"I have been boxing for 12 years, I have been trying to play in the Olympic Games," said the mother of two. "Today is very emotional. Today is my twins' birthday and I can't celebrate with them. But I am fighting in the ring and winning. That will be a gift for them."
International Boxing Association (AIBA) president Wu Ching-kuo called the first day of women's boxing at the Olympics "an historic moment".
"I am very proud today to witness the first women boxers taking in an Olympic ring. It is an historical moment, not only for AIBA, but for the Olympic Movement in general," said Wu.
"We have put a lot of effort into making this happen and now we are very excited to see that our dream has become a reality."