A knee injury that has bothered No. 2-ranked Li Na since spring has now forced her to withdraw from the U.S. Open.

Na is the only tennis player from China to win a Grand slam singles title —  she has two. Her announcement on Thursday came by way of a Facebook posting.

Writing that the knee has been sore since March, Na added that she is "just not where I need to be in order to play at the highest level."

Her right knee was taped through the early rounds at Wimbledon this summer. She wrote: "My medical team has advised me that I need to take some time off to rest my knee."

Earlier this week, Rafael Nadal cast doubt on whether he will play in the Open in late August, withdrawing from hard-court tournaments in Toronto and Cincinnati because of a sore wrist.