Great Britain struggled against three-time Olympic silver medallists Australia in the opening game of their Olympic debut, losing 74-58.
A strong start by GB seemed to rattle the Opals but Australia's experience soon shone through and they went in 39-26 up at half-time.
Reigning Olympic champions USA stuttered early on in their game against Croatia but came through 81-56.
European champions Russia beat Canada 58-53, and France beat Brazil 73-58.
African champions Angola, who were also making their Olympic debut, were comprehensively beaten 72-50 by Turkey, while China beat the Czech Republic 66-57.
Britain continue their Group B campaign against Canada on Monday.
United States player Sue Bird admitted her team need to improve despite winning their opening game against Croatia.
The gold medal favourites led just 53-47 going into the final quarter before finally hitting form to make the scoreline more emphatic.
"A win is a win and we're going to take it, learn from it and hopefully get better for the next game," said Bird.
The victory extends the US's Olympic winning streak to 34 games.
But seven-time WNBA all-star Bird, 31, insists the US women must be at their best if they are to win a fifth straight gold.
"We're happy about our chances - we think they are good - but it takes just one bad game for one team, one great game for another and anything can happen so you've got to be ready," she told BBC Sport.
"Our standards are high for ourselves. You look at our turnovers, the fact we had a team on the ropes early and didn't put them away, and those things make me believe we didn't have a great performance."
Bird is appearing at her third Olympics, after winning gold in Athens and Beijing, and insists the squad is hungry for success.
"This team hasn't won anything and that makes us a very hungry team," she added. "This is a team that wants to experience that gold medal - wants to have it in our hands or around our necks.
"For some on this team, they haven't experienced it; I'd like to help give them that opportunity. I know we're all fighting for the same thing."