Serena Williams has pulled out of the BNP Paribas Open at Indian Wells, meaning she will lose her position as world No 1.
Williams has not played since victory at January's Australian Open brought a 23rd Grand Slam singles title and saw her return to the top of the rankings.
And a problem with her left knee will sideline the 35-year-old for Indian Wells and the subsequent WTA Tour event in Miami.
"Sadly, I have to withdraw from the BNP Paribas Open in Indian Wells and the Miami Open," she said.
"I have not been able to train due to my knees and am disappointed I cannot be there. I will keep moving forward and continue to be positive. I look forward to being back as soon as I can."
Angelique Kerber will now reclaim the No 1 spot from Williams, who needed to reach the semi-finals in Indian Wells to retain her position.
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