British pair John Garcia-Thompson and Steve Grotowski were beaten by Canada's Joshua Binstock and Martin Reader in their beach volleyball opener.
Garcia-Thompson and Grotowski, GB's first ever male representatives at an Olympics, lost 2-0 (21-19, 21-13).
USA duo Kerri Walsh and Misty May-Treanor made a winning start to their bid for a third straight gold medal.
The pair defeated Australia's Nat Cook and Tamsin Hinchley 2-0 (21-18, 21-19) on a floodlit Horse Guards Parade.
Six-time world champions Larissa Franca and Juliana Silva of Brazil eased past Natacha Rigobert and Elodie Li Yuk Lo of Mauritius in straight sets (21-5, 21-10).
Britain's women Zara Dampney and Shauna Mullin start their campaign on Sunday when they face the Canadian pairing of Anni Martin and Marie-Andree Lessard.
GB's men are next in action on Monday when they meet Brazil's Ricardo Santos and Pedro Cunha, who overcame Norway's Tarjei Viken Skarlund and Martin Spinnangr 2-0 (21-14, 21-18).