Venus Williams has been forced to pull out of the WTA Tour's upcoming Western and Southern Open in Cincinnati because of a viral illness.
The 31-year-old American missed this week's Rogers Cup in Toronto because of 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."
It is the second successive year that the world number 34 has missed the Western and Southern Open after she pulled out last year because of a knee injury.
Williams has played only three tournaments this season due to injury and illness.
She faces a race against time to be fit for the US Open, which starts on Monday, August 29.