Two-time major champion Li Na is pulling out of the US Open because of a knee injury.

The only tennis player from China to win a Grand Slam singles title is also withdrawing from hard-court events in Montreal and Cincinnati over the next two weeks.

Li said, in a posting on her Facebook page, that she has "been struggling'' with her knee since March "and it is just not where I need it to be in order to play at the highest level".

The US Tennis Association tells The Associated Press that Li informed the tournament she will not play. The US Open begins August 25.

Li won the Australian Open in January for her second major title. She also won the 2011 French Open.

Source: AP