World No.1 Serena Williams has pulled out of Cincinnati due to a right shoulder injury.
The same injury saw the the two-time defending champion pull out of the Rogers Cup in Montreal last month.
"I'm very disappointed I'm not able to compete in the Western & Southern Open in Cincinnati, as I was really looking forward to defending my title," Williams said in a statement.
"My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible."
The withdrawal means Williams could lose her No.1 ranking if Angelique Kerber wins the tournament in Cincinnati.
She has held the No.1 rankings for 183 weeks, second all time to Stefi Graf's record of 186 weeks.
The 34-year-old's next event will be the US Open, which begins on August 29.