Cincinnati - 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."