The U.S., which took silver four years ago in Beijing, and gold medal-favorite Serbia are tied for the Group B lead with four points apiece after two matches. Serbia beat host Britain 21-7.
Montenegro, Croatia and Australia also won. Greece and Italy played to a 7-7 tie.
Shannon Taylor scored late in the first half and the U.S. upset Argentina 1-0 in women's field hockey.
The Americans controlled long stretches of play against the No. 2-ranked team in the world, got some key saves from Amy Swensen and kept Argentine star Luciana Aymar largely silent throughout.
The U.S. got into the Olympics by stunning Argentina 4-2 in last year's final at the Pan American Games.
Netherlands, New Zealand, Britain and Australia also won. China and Belgium played to a scoreless tie.
Vincent Hancock is putting together quite the Olympic resume. The 23-year-old U.S. Army sergeant is a two-time champion in men's skeet shooting after he successfully defended his crown with a score of 148 in London.
Hancock's win gave the U.S. a skeet sweep after Kimberly Rhode won the women's competition earlier this week. Anders Golding of Denmark grabbed the silver, and Qatar's Nasser Al-Attiya won a shoot-off over Russia's Valery Shomin for the bronze at the Royal Artillery Barracks.