Serena Williams defeated Samantha Stosur of Australia 6-2, 6-0 in 44 minutes.
Sister Venus soon joined Serena in the third round, beating Iveta Benesova of the Czech Republic 6-1, 6-4.
The Williamses are trying to add to the family medal collection. They won the gold in doubles at Sydney in 2000, and Venus won the singles that year.
“I’ve never played singles at the Olympics, so this has been really cool for me,” Serena said. “Every time I walk out there, it’s like I’m playing in my match, but at the same time I have the whole U.S. team on my side. It’s good. I really like that feeling.”
The sisters are top-seeded in doubles and had a first-round match scheduled Tuesday night. They could meet in the final of singles Saturday.
“That would be fantastic,” Venus said. “Obviously it’s a long ways away, but the third round makes it closer. I’m really excited for both of us to be playing really well for the U.S.”
Information from the Associated Press contributed to this report.
