Stosur, capitalizing on an unexpected day of rest, returned to the court Thursday and upset fifth-seeded Li Na of China 6-4, 3-6, 6-4 in the U.S. Open tuneup.
The 10th-seeded Australian was scheduled to face Williams on Wednesday in the round of 32 before Williams withdrew after aggravating a right big toe injury.
"I wasn't complaining that (Williams) pulled out," said Stosur, who lost to Williams in the finals at Toronto last week. "I had a pretty easy day, which was nice, considering the week I had before. So I guess that was good for today's match and hopefully will be good for tomorrow."
Stosur has beaten Li -- the French Open champion -- three times this season, including in the same round last week at Toronto.
Stosur became the first woman to reach the quarterfinals. Slovakia's Daniela Hantuchova followed when she upset eighth-seeded Marion Bartoli 6-3, 5-7, 6-3. Hantuchova improved to 5-1 all-time against Bartoli. Second-seeded Vera Zvonareva of Russia also advanced to the quarterfinals with a 6-2, 6-2 win over Petra Martic, as did fourth-seeded Maria Sharapova, who beat Svetlana Kuznetsova 6-2, 6-3.
Also Thursday, ninth-seeded German Andrea Petkovic beat No. 6 Petra Kvitova of the Czech Republic 6-3, 6-3; No. 13 Jelena Jankovic of Serbia held off seventh-seeded Italian Francesca Schiavone 6-3, 5-7, 6-4; and No. 16
Peng Shuai of China beat Israel's Shahar Peer 6-3, 6-2.
Information from The Associated Press was used in this report.