Western and Southern Open defending champion Victoria Azarenka has withdrawn from the tournament in Cincinnati after sustaining an injury on her right knee on Tuesday.

The World Tennis Organization (WTA) said Azarenka withdrew after practicing Tuesday morning and was replaced by Mona Barthel.

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Azarenka, a two-time Australian Open champion, said she injured her knee during the first round at the recent Wimbledon.

Azarenka went on to play in two other tournaments, including the Rogers Cup in Montreal last week where she lost in the quarterfinals against eventual winner Agnieszka Radwanska that further aggravated her right knee.

Azarenka said she felt a little better after some rest, but "It's just not enough time for me to feel good to play a full match."

''Unfortunately I won't be playing in Cincinnati this year. But I had to make the best decision for me. I will definitely be back next year,'' she posted on her Twitter account.

The 25-year-old had won the event last year against World No. 1 Serena Williams in a thrilling three-setter 2-6, 6-2, 7(8)-6(6).

Barthel, 25, replaced the defending champ in the tournament and faced Barbora Zahlavova-Strycova in the first round but eventually lost in two sets 5-7 0-6.

Azarenka will now have some time to rest and get some treatment on her injured knee and hopefully on time for the U.S. Open at Flushing Meadows slated to start on Aug. 25.

The former World No. 2 has not won any WTA tournaments this year and just made the quarterfinals in Montreal compared to the stellar performance last year where she won the Australian Open.

She was also a semifinalist at the French Open, and also a was runner-up to Williams at the U.S. Open.

Azarenka will go down in the world rankings to No. 16 starting Monday.

TagsVictoria Azarenka, Barbora Zahlavova-Strycova, serena williams, Mona Barthel