Defending champion Victoria Azarenka was forced to withdraw on Tuesday from the women`s event at the ATP-WTA Cincinnati Masters with a right knee injury.
Officials said the tenth seed will be replaced in the draw by German Mona Barthel who plays later against Barbora Zahlavova Strycova.
Due to a foot injury Azarenka, owner of two Australian Open titles, has played just six events in 2014. Her best showings were quarter-finals in Melbourne and last week in Montreal.
"After my last match in Montreal, I tried to recover as much as possible. I started to feel a little bit better and did everything I could, but it`s just not enough time for me to feel good to play a full match," said Azarenka.
"I tried to practise, give it a rest and work with my team. It`s just not enough time. I wanted to give it my best shot until the last moment and try to do the best job as possible.
"It`s unfortunate but it`s the best decision for me."
Azarenka may have taken her decision with the US Open start in less than a fortnight in mind.
"I`m going to get more treatment and get more work done and should be good for the US Open. But I had to make this decision. I feel strongly that it`s the best for me at this moment."