The 32-year-old Li, who won her second grand slam title at this year's Australian Open, was knocked out in the first round of the French Open and in the third round at Wimbledon.
"Since March, I have been struggling with my knee and it is just not where I need it to be in order to play at the highest level," Li said on Facebook.
"My medical team has advised me that I need to take some time off to rest my knee so it heals," the 2011 French Open champion added.
Li hopes to return to action by late September.
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