The three-time champion overcame a sluggish start and wobbly finish Monday to beat No. 10-seeded Agnieszka Radwanska 4-6, 6-1, 6-4.
Williams, seeded fifth, nearly blew a 5-1 lead in the final set and double-faulted three times in the last game before closing the victory with her 11th ace.
Six of the top 10 women have been eliminated, including No. 4-seeded Elena Dementieva, who lost her fourth-round match to 18-year-old Caroline Wozniacki 7-5, 6-4.
On the men's side, Rafael Nadal reached the fourth round by beating qualifier Frederico Gil 7-5, 6-3.
