Monday, August 15, 2011
It is the second consecutive year the American has withdrawn from the event as she skipped the 2010 tournament with a sore knee.
The world No. 34 was forced to withdraw from this week's Rogers Cup in Toronto with the same illness.
"I regret not having the opportunity to compete in the Western and Southern Open this year," Williams said. "I am making progress towards recovering from a viral illness that began affecting me last week, and have plans to return to the practice court in the coming days. I am optimistic that I will be ready for the US Open at the end of this month."
The 31-year-old American has been limited to three tournaments this year because of injuries and illness. She retired at the Australian Open in January because of a right hip injury and contended with hip and abdominal ailments through May.