MELBOURNE, Australia -- Serena Williams withdrew hours before her opening match at the Australian Open, citing an ankle injury she picked up in a tuneup event.

The fifth-seeded Williams was scheduled to play Conchita Martinez later Monday on a show court at Melbourne Park.

First-round matches had started on outside courts when tournament officials announced Williams' withdrawal -- not long after defending men's champion Andre Agassi pulled out with an injured wrist. Women's No. 1 Lindsay Davenport already was sidelined after right knee surgery Friday.

Williams twisted her ankle chasing a drop shot in a semifinal Friday against Meghann Shaughnessy at the Adidas International in Sydney. Williams continued playing for three games against Shaughnessy before retiring when trailing 5-4 in the first set.

Williams, the 1999 U.S. Open champion, was in a tough portion of the Australian Open draw along with her sister Venus and former No. 1 Martina Hingis.

Williams lost to Hingis in the quarterfinals here last year.