China's world number two Li Na has been ruled out of this month's US tennis Open with a knee injury.
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.
She says since March, she's been struggling with her knee and it's not where she needs it to be in order to play at the highest level.
Li says her medical team has advised her that she needs to take some time off to rest the knee so it heals.
