The U.S. women's team beat Brazil in an early rematch of the Beijing Games final.
Destinee Hooker had 23 points and Jordan Larson added 18 for the top-ranked Americans, who won 25-18, 25-17, 22-25, 25-21 to improve to 2-0 in pool play at Earls Court.
Sheilla Castro had 15 points for No. 2 Brazil, which was energized after a third-set victory but lost on Logan Tom's floater in the fourth set.
The American women have never won an Olympic gold medal in volleyball.
China, South Korea, Russia and Italy also posted four-set victories, and Britain edged Algeria in five.
Two-time gold medalists Misty May-Treanor and Kerri Walsh Jennings earned their 16th consecutive Olympic victory, beating the Czech Republic in straight sets.
It was May-Treanor's 35th birthday when the match started, but because of TV schedules and a long match earlier in the session, it ended shortly after midnight Tuesday.
Jake Gibb and Sean Rosenthal lost to Poland in pool play, the first setback for the American men or women in beach volleyball at the games.
Zara Phillips, Queen Elizabeth II's granddaughter, raced through the difficult and dangerous cross-country portion of her first Olympic equestrian eventing competition, finishing clean and well under the pace time.
Princes William and Harry watched her from the main equestrian arena, joined by William's wife, Kate, and Camilla, the wife of Prince Charles. Seated alongside them were Princess Beatrice and Princess Eugenie, the daughters of Prince Andrew.
Phillips' mother, Princess Anne, watched from the grounds of the twisty, hilly 3.5-mile course dotted with 28 obstacles.
Several of the riders who fell wore protective vests that inflated much like airbags upon impact. One of them, Hawley Bennett-Awad of Canada, was in the hospital under observation for a concussion and fracture to a bone at the base of the spine. There were no other serious injuries reported among the other fallen riders or horses.
This was quite the Olympic debut for Maggie Steffens, who scored seven goals to lead the U.S. women's team to a 14-13 victory over Hungary.
Despite a team full of veterans, it was the 19-year-old Steffens who led the way with sharp shooting from outside for the Americans, who are looking to win their first gold in the event.
Russia spoiled Britain's Olympic debut in women's water polo, getting a late breakaway goal from Evgeniya Ivanova in a 7-6 victory. Spain and Australia also won their matches.
China is dominating the diving boards — again.
Cao Yuan and Zhang Yanquan totaled 486.78 points in the men's 10-meter synchronized platform, giving the country its second gold medal in the sport at the games.
German Sanchez and Ivan Garcia of Mexico had the highest degree of difficulty in the competition and it paid off with the silver.
Nick McCrory and David Boudia of the U.S. took the bronze with 463.47. The Americans are 2 for 2 after Abby Johnston and Kelci Bryant earned a silver in 3-meter synchro springboard Sunday, ending a 12-year medal drought.
Britain's Tom Daley and Peter Waterfield led through the first three rounds, but botched their fourth dive and finished fourth.
Fencer Yana Shemyakina of Ukraine beat defending champion Britta Heidemann of Germany 9-8 to win the Olympic gold medal in women's individual epee.
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