SAN JOSE, CA, USA - No.2 seed and defending champion Madison Keys was forced to withdraw before taking the court at the Mubadala Silicon Valley Classic on Thursday with a right wrist injury.

The American was the highest remaining seed in San Jose following Wednesday's withdrawal of top seed Garbiñe Muguruza, and was slated to play Australia's Ajla Tomljanovic in the second round. 

"I am really sorry to have to pull out of the Mubadala Silicon Valley Classic," said Keys. "I have been feeling pain in my wrist over the last couple of days and felt worse today.

"This event is one of my favorites and I was looking forward to playing here in San Jose, but want to thank the San Jose State University and Mubadala for pulling it together in such a short amount of time. I am excited to come back next year and play in front of your great fans."

Keys is replaced in the draw by lucky loser Magdalena Frech of Poland, who will face Tomljanovic.

The winner will advance to face No.5 seed Mihaela Buzarnescu, who booked her place in the last eight by winning the first match on Thursday's schedule

No.3 seed Venus Williams, now the highest-ranked player in the draw, will kick off Thursday's night session against Great Britain's Heather Watson.