Defending champion Victoria Azarenka has pulled out of the Western and Southern Open in Cincinnati because of a knee injury.
The Belarusian played only one match in four and a half months between January and June because of a foot injury and has won just 10 matches this year.
She had progressed to the quarter-finals in Montreal a week ago, but hurt her right knee in her opening match against Alize Cornet and has decided not to risk it in Cincinnati.
Azarenka, who is replaced in the draw by German lucky loser Mona Barthel, beat Serena Williams in the final 12 months ago.
This injury has also placed her US Open ambitions in doubt and is likely to see her lose her place in the top 10 of the ATP's global rankings.