By Adrianna Outlaw

(October 10, 2010) Don't call it a comeback yet. Serena Williams has delayed her planned return to tournament tennis yet again after aggravating her foot injury in a training session. The 13-time Grand Slam champion has stepped out of next week's Generali Ladies Linz event due to the foot injury, tournament organizers announced today.

Williams, who has not played a match since she beat Vera Zvonareva to win Wimbledon in July, felt the foot injury during a training session in Florida yesterday and opted to pull out of Linz.

"It was discovered that the tendon injury which has affected her after her Wimbledon victory is still not completely healed and Williams must therefore withdraw," Linz tournament organizers announced in a statement issued today. "How long she will be sidelined is still unclear."

Williams had requested a visa to enter Russia in apparent preparation to play the Kremlin Cup in Moscow, starting on October 18th, but her status for the Moscow event is now uncertain.

Caroline Wozniacki will supplant Serena as the new World No. 1 when the new WTA Tour rankings are released on Monday.