Defending champion Madison Keys has been forced to withdraw from the Silicon Valley Classic with a right wrist injury.

With Garbine Muguruza already having retired, Keys was the highest seed remaining in the WTA Premier tournament as she prepared for Thursday's meeting with Ajla Tomljanovic.

But hours before that match, the world number 12 followed in Muguruza's footsteps and pulled out.

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

"This event is one of my favourites and I was looking forward to playing here in San Jose, but want to thank 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."