The sixth-seeded Williams beat Francesca Schiavone, 3-6, 6-2, 6-1, early today, recovering after dropping a set to the Italian for the first time since 2003.
Along with 2008 Wimbledon semifinalist Zheng Jie, who is on the other half of the draw at Melbourne Park, it's the first time two Chinese players have reached the quarterfinals at the same Grand Slam tournament.
Williams reached the final of the Australian Open in '03, losing to her younger sister Serena, in her best run at the season's first major.
The Williamses are on track to meet in the semifinals here, with defending champion and No. 1-ranked Serena playing in a fourth-round match later Monday against Australia's Sam Stosur.
Williams' win extended a streak in which at least one American woman has reached the Australian Open quarterfinals every year since 1977.
Roddick held off the 2007 Australian Open finalist, 6-3, 3-6, 4-6, 7-5, 6-2, Sunday to advance to the quarterfinals for the sixth time in eight years.
The 27-year-old American will meet No. 14 Marin Cilic of Croatia, who ousted U.S. Open champion Juan Martin del Potro in five sets.
Roddick bucked the trend on a day of upsets that also resulted in fourth-round exits for two of the top women: No. 2 Dinara Safina, last year's runner-up, and No. 3 Svetlana Kuznetsova, the reigning French Open champion.
"I got a little lucky tonight, but sometimes it's better to be lucky than good," said the seventh-seeded Roddick, who hopes to end a drought in majors that dates to the 2003 U.S. Open.
Safina had to retire because of the recurrence of a back injury when she was serving at 5-4 down and 30-40 in the first set against Maria Kirilenko.
Former No. 1 Justine Henin continued her run in her first Grand Slam tournament in two years with a 7-6 (3), 1-6, 6-3 win over fellow Belgian Yanina Wickmayer, keeping her on track for a quarterfinal meeting with Petrova.
In an afternoon match, 2008 Wimbledon semifinalist Zheng Jie became the first Chinese player to reach the quarterfinals at Melbourne Park with a 7-6 (6), 6-4 victory over Alona Bondarenko.
Rafael Nadal had a 6-4, 4-6, 6-4, 6-4 win over 6-10 Ivo Karlovic of Croatia, and Andy Murray overcame 6-9 American John Isner, 7-6 (4), 6-3, 6-2, to reach the final eight in Australia for the first time.