Defending champion Victoria Azarenka has been forced to pull out of the Cincinnati Open due to a knee injury.
The Belarusian suffered the problem last week during her run to the quarter-finals of the Rogers Cup in Montreal.
German world number 44 Mona Barthel has replaced the two-time Grand Slam champion in the main draw.
Barthel is due to face Barbora Zahlavova-Strycova in the opening round of the tournament.
