Although the two players had faced each other a few times in doubles, it was their first encounter in a singles match.
Williams will be participating in her fourth semifinal of the year after Oakland, Dubai and the Rogers Cup in Montreal.
Fifth seed Julia Goerges topped Andrea Hlavackova in a 6-7, 7-5, 6-2 quarter-final win.
Mirjana Lucic-Baroni eliminated Sesil Karatantcheva with a 7-5, 6-0 victory.
Shelby Rogers had no trouble beating Tatjana Maria 6-4, 6-2.