Serena Williams was ranked No. 304 when she beat Mary Pierce and Monica Seles in Chicago in 1997.
The achievement came at the Ameritech Cup in November 1997, only the second WTA Tour main-draw event of Williams's career. In the second round, the 16-year-old defeated No. 7 Mary Pierce, then shocked No. 4 Monica Seles in the quarterfinals, 4–6, 6–1, 6–1.
Williams's run was historically significant because she became the lowest-ranked player in the Open Era to defeat two Top 10 opponents in one tournament. She reached the semifinals before losing to Lindsay Davenport, but the Chicago results propelled her into the world's Top 100 for the first time.
A common mix-up is No. 99: that was Williams's year-end ranking for 1997, not her ranking during the Chicago tournament.