Two-time champion Victoria Azarenka withdrew from the Sony Open before her opening match on Friday because of a lingering right ankle injury.
The third-ranked Belarusian was replaced by lucky loser Lauren Davis of the United States.
The same injury forced Azarenka to withdraw before her quarterfinal match a week ago at Indian Wells because of inflammation and tendinitis.
Earlier this week, the third-ranked Azarenka was optimistic she would play. On Friday, she released a statement saying she's "been working very hard to rehab" her ankle, but "it is still not fit to play."
Azarenka won the tournament in 2009 and 2011.
Azarenka has a 17-0 record this year with two titles. She's withdrawn from three tournaments, including the Australian Open before the semifinals because of a right toe injury.
NGUYEN: ATP without Federer, Nadal