Serena Williams will relinquish her world number one tennis ranking to German Angelique Kerber after withdrawing from the Indian Wells Masters and Miami Open due to a left knee injury.
Williams, a two-time champion at the showpiece event in the California desert, returned to the top ranking in January after claiming a record 23rd grand slam singles title at the Australian Open.
The women's main draw at Indian Wells begins on Wednesday. The Miami Open, which Williams has won eight times, runs from March 20-April 2.
"Sadly, I have to withdraw from the BNP Paribas Open in Indian Wells and the Miami Open," Williams said in a statement on Tuesday.
"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."
Williams, 35, had needed to reach the Indian Wells semi-finals for a chance to retain her number one ranking, but her withdrawal means Kerber will unseat her after the tournament, according to the WTA website.
In 2015, Williams returned to Indian Wells for the first time since an ugly episode in 2001 when she triumphed in a final marred by racist abuse from some spectators who booed and heckled the American and her family.
The heckling was in apparent response to Serena's sister Venus having withdrawn from their semi-final that year just minutes before the match.
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