China, South Korea, Russia and Italy also posted four-set victories, and Britain edged Algeria in five.
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.