BEIJING — Todd Rogers and Phil Dalhausser, the defending beach volleyball world champions and the overwhelming favorites to win the Olympic gold medal, lost their opening match on Saturday night to a Latvian team that was seeded 23rd in the 24-team field.

The Americans, who had won 21 straight international matches, must win their next two matches in pool play to be assured of a spot in the medal round.

Martins Plavins and Aleksandrs Samoilovs, former junior world champions who are the first Latvian team to qualify for the Olympics, won 21-19, 21-18 in the best-of-three set match.