Brazil went top of Group E as they opened with a comfortable 5-0 victory over tournament newcomers Cameroon at the Millennium Stadium in Cardiff.
Early goals from Francielle and Renata Costa put the Samba Queens in control, before captain Marta with a brace and substitute Cristiane sealed victory.
USA, gold medallists at the Beijing 2008 Games, came from behind to edge out France 4-2 in a thrilling Group G contest, with Alex Morgan scoring twice.
Abby Wambach and Carli Lloyd were also on target for USA after Gaetane Thiney and Marie-Laure Delie had given France a two-goal lead.
Meanwhile, world champions Japan opened their account with a 2-1 win over Canada in Group F.