Australian defending champions Natalie Cook and Kerri-Ann Pottharst made a promising start with different partners in the beach volleyball on Saturday.
Sanderson helped Cook to upset the Bulgarian pair on Saturday
Pottharst and Summer Lochowicz, seeded 18th, upset seventh seeds Jia Tian and Wang Fei of China 21-18 21-18.
Number five seeds Cook and Nicole Sanderson beat Bulgarian sisters Lina and Petia Yanchulova 21-16 21-12.
Cook and Pottharst won gold in Sydney four years ago before ending their eight-year partnership in 2003.
Pottharst survived an auspicious beginning to her Olympic campaign, when partner Lochowicz slipped in the bathroom and skinned her knee moments before they took to the court.
But once the match started, it was Tian and Wang who stumbled, faulting on 14 serves and losing 27 of 46 points decided at the net.
"The first match can be nerve-racking. It's good to get that one out of the way," Pottharst said afterwards.
Elsewhere, Americans Holly McPeak and Elaine Youngs, seeded fourth, cruised past 21st-seeded Norwegians Susanne Glesnes and Kathrine Maaseide 21-14 21-14.
Canadians Guylaine Dumont and Ann Martin upset the ninth-seeded Swiss pair of Simone Kuhn and Nicole Schnyder-Benoit 21-16 21-13.
Among the highlights of the men's fixtures on Saturday, Australian duo Julien Prosser and Mark Williams cruised past Americans Dain Blanton and Jeff Nygaard 21-16 21-14.
Blanton, who teamed with Eric Fonoimoana to upset the Brazilian favourites for the gold in Sydney, failed to make a winning combination with new partner Nygaard.
"I don't feel like I have a good offensive synch right now," said Nygaard.
"We gave them no pressure because I wasn't blocking anything."
Elsewhere, South Africa's Gershon Rorich and Colin Eric Innes Pocock survived a fightback from Greek hopefuls Pavlos Beligratis and Athanasios Michalopoulos to win 21-16 24-26 15-10.
Mariano Baracetti and Martin Conde of Argentina came from behind to beat Portugal's Miguel Maia and Joao Brenha 13-21 21-16 15-5.