Serena Williams withdrew from the Dubai Championships because of a lower back injury just minutes before she was due on court for a second round match against Frenchwoman Marion Bartoli.
The American's withdrawal means that the world's two best players are both out of the tournament after Victoria Azarenka, who beat Williams in the Qatar Open final at the weekend, also pulled out earlier in the week with a foot injury.
Williams, the newly installed world number one, aims to return at the Sony Ericsson Open, which starts in Miami on March 18th.
31-year-old Williams says her back also troubled her in last week's Qatar Open.
The American's problems appear to stem from January's Australian Open, where she suffered a shock quarter-final loss to U.S. teenager Sloane Stephens, having been forced to briefly leave the court for medical treatment after jarring her back.
Bartoli will now play former world number one Caroline Wozniacki in the quarter-finals after the Dane routed Chinese qualifier Zheng Jie 6-love 6-1.
