Beach volleyball pair Misty May and Kerri Walsh took a 21-11 21-13 win over Rebekka Kadijk and Marrit Leenstra.
The American women, who finish their group against Czech pair Eva Celbova and Sona Novakova on Thursday, look set to challenge for medals.
But their male compatriots Dax Holdren and Stein Metzger are on the brink of elimination after a 14-21, 21-15, 16-14 loss to Iver Horrem and Bjorn Maaseide.
All four Brazil teams stayed unbeaten and clinched berths in the medal round.
"We blew it," said Metzger after his loss.
"We had a lot of opportunities, but we just couldn't finish in the second and third games."
Brazilian women's pair, Shelda Bede and Adriana Behar - the 2000 silver medallists in Sydney, edged past Italian pair Daniela Gattelli and Lucilla Perrotta 21-17 21-17.
And Bede paid tribute to the loud support in Athens.
"The atmosphere in the stadium was very important to us. The Brazilian fans kept us going."