REUTERS: Holder Madison Keys withdrew from the Silicon Valley Classic on Thursday ahead of her second round match against Australian Ajla Tomljanovic due to pain in her right wrist.
The hard-hitting 23-year-old, runner-up at last year's U.S. Open where she lost to fellow American Sloane Stephens, is ranked 12 in the world and was the second seed in San Jose.
"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..." she added.
Keys will be replaced in the draw by Polish lucky loser Magdalena French who will face Tomljanovic on Thursday.
Keys was the highest remaining seed after Spaniard Garbine Muguruza withdrew on Wednesday due to a right arm injury.
(Reporting by Rory Carroll; Editing by Ken Ferris)