California - Venus Williams beat Heather Watson 6-4, 4-6, 6-0 on Thursday night in the second round of the Mubadala Silicon Valley Classic.
The third-seeded Williams is the highest-seeded player remaining in the event after defending champion Madison Keys joined top-seed Garbine Muguruza, a Wednesday scratch, on the sideline due to a right wrist injury.
Keys, an American, was slated to play Australia's Ajla Tomljanovic, a 6-4, 3-6, 6-3 winner over lucky-loser Magdalena Frech of Poland.
"I have been feeling pain in my wrist over the last couple of days and felt worse today," Keys said in a statement.
Romanian fifth-seed Mihaela Buzarnescu beat 16-year-old American qualifier Amanda Anisimova 7-5, 2-6, 6-1 to reach her seventh WTA quarter-final of the season.
Buzarnescu rallied from a 5-3 deficit in the opening set, and broke Anisimova's serve to start the third set en route to a 4-0 lead.
Maria Sakkari of Greece won 11 straight games against Hungarian Timea Babos, winning 6-0, 6-1. Sakkari will play Williams in the quarter-finals.
Results from WTA San Jose tournament on Thursday (x denotes seed):
Venus Williams (USA x3) bt Heather Watson (GBR) 6-4, 4-6, 6-0
Maria Sakkari (GRE) bt Tímea Babos (HUN x8) 6-0, 6-1
Mihaela Buzarnescu (ROM x5) bt Amanda Anisimova (USA) 7-5, 2-6, 6-1
Ajla Tomljanovic (AUS) bt Magdalena Frech (POL) 6-4, 3-6, 6-3