(CNN) -- World No. 2 Li Na has pulled out of next month's U.S. Open with a knee injury.
Li, the reigning Australian Open champion and a semifinalist at Flushing Meadows last year, posted a message on her Facebook page saying she expects to return to action after the tennis season's fourth and final grand slam event.
"I wanted to let all of my great fans know that unfortunately I am going to have to withdraw from the upcoming WTA events in Montreal and Cincinnati, as well as the U.S. Open," Li's statement read.
"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.
"My medical team has advised me that I need to take some time off to rest my knee so it heels. I look forward to getting back on the court this fall in Wuhan and Beijing."
After the two Chinese tournaments in late September, there is just one premier-level WTA Tour event before the season-ending championships in Singapore starting October 20. Li is third in the standings for the eight-woman showpiece.
Injury to her right knee forced Li to withdraw from April's Stuttgart Open, and the 32-year-old has struggled with her form since.
A first-round exit at Roland Garros in May was followed by a third-round defeat at Wimbledon the following month, which is her last tournament appearance.
Li became China's first grand slam singles champion when she won the French Open in 2011 and clinched a second major title at Melbourne Park in January, when she beat Slovakia's Dominika Cibulkova in straight sets.
Thursday's announcement came 24 hours after Rafael Nadal revealed he is suffering from a wrist injury and may not be fit for the New York slam.
Nadal clinched his second U.S. Open title against Novak Djokovic last September but his title defense could be in peril after he was told by doctors that he needs at least two to three weeks off from competitive action.
The injury, sustained during practice, has denied the second-ranked Spaniard the chance to defend his titles at next week's Rogers Cup in Toronto and the Cincinnati Masters, which starts on August 10.