"We started bad. We lost the first set but we kept our mind on the match, tried to keep calm," said Adriana, who triumphed with Shelda 15-21 21-13 15-13.
"If you start to be crazy you can't keep in the match."
Adriana and Shelda -- the shortest woman in the tournament but one of the most agile -- have been playing together since 1995 and won the silver medal at the Sydney Olympics.
American pair set for Brazil clash
The result meant that for the first time Brazilian women won't take home two Olympic medals.
Meanwhile, the U.S. women are placed to win not just their first Olympic medal but could win two.
Top-seeded Misty May and Kerri Walsh will face compatriots Holly McPeak and Elaine Youngs in a semi-final on Monday after winning their matches on Sunday.
"We're so familiar with them. They are such an unbelievable team," said Walsh.
"We're going to have to play our best volleyball tomorrow to beat them, and we plan on it."
Walsh said she and May, who struggled in the first set against Canada's Guylaine Dumont and Annie Martin before winning 21-19 21-14, needed to concentrate and stay in control to beat their fourth seeded compatriots.
McPeak and Youngs beat Germany's Okka Rau and Stephanie Pohl 21-17 21-17 to reach the next round.
Olympic champion Natalie Cook, who defeated Shelda and Adriana for the gold medal in Sydney four years ago, and new partner Nicole Sanderson beat Italy and will face the Brazilians in the semi-finals.
Teams playing during the day said their energy was sapped by temperatures as high as 42 Celsius (107.60F) on the sand.
In the men's competition, Daxton Holdren and Stein Metzger of the United States, who lost to Switzerland's Patrick Heuscher and Stefan Kobel 21-16 21-19, said the heat and the burning sand hurt their game.
Heuscher agreed, pointing to a long rally in the second set which left all four players lying on the sand gasping for air.
"We killed each other," he said. "I didn't understand the ref -- why he wanted to keep us playing and playing, which is just not possible in this heat. You need some rest because your heart is pumping like hell."
Heuscher and Kobel will next play Brazil's top-seeded Ricardo Santos and Emanuel Rego who beat Paul and Martin Laciga of Switzerland 21-13 21-16.
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