Despite pulverising the Czech Republic 79-46, the world champions were left sweating on the condition of guard Penny Taylor, who hobbled out of the game with an ankle injury.
Taylor collapsed in agony after twisting her right ankle on a drive to the basket in the third quarter and is a doubt for Thursday's semi-finals.
The Olympics hosts China await Australia in the last four after overpowering Belarus 77-62.
Australia's chances of denying fierce rivals the United States a fourth successive Olympic title would be significantly diminished without Taylor, the team's best defensive player.
"Penny's a huge part of our team," Opals captain Lauren Jackson told Reuters. "We've got to hope for the best. I don't know what we're going to do without her.
"We've got the best doctors and physios in the world so I'm sure they'll get it right. But we're going to go out there and give it our best shot regardless."
Australian doctors were hopeful that Taylor, who leads the team in steals, could still play a part in the Beijing tournament.
"She's got a pretty significant injury. We'll hopefully see her back on the court if not for the semi-final then hopefully for the final," team doctor Scott Burne said.
MIAO ROARS
Chinese captain Miao Lijie, the home team's best player during the group stage, scored 28 points to keep alive her team's chances of a first Olympic medal in 16 years.
"I'm relieved," said China's Australian coach Tom Maher. "After we beat the Czechs and Spain [in the first round] there's no way we should lose to Belarus -- no offence.
"If you win the quarter-final the rest is gravy. But I just want to beat Australia -- China is where my passion lies."
The US, bidding for a fourth straight Olympic title, Australia and Russia are expected to fill the podium places in Beijing.