NEW YORK - Two-time major champion Li Na pulled out of the US Open on Thursday because of a knee injury which she said has bothered her for several months.

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

The second-ranked Li wrote in a posting on Facebook 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.''

She also said: ``My medical team has advised me that I need to take some time off to rest my knee.''

Li did not specify which knee, but she played with her right knee taped up at Wimbledon, her most recent tournament.

The US Tennis Association confirmed to The Associated Press that Li informed it that she won't play. The US Open begins on Aug. 25.

Li won the Australian Open in January for her second major title. She also won the 2011 French Open. In May, Li lost in the first round of the French Open, beaten by a player ranked 103rd. At Wimbledon in June, Li bowed out in the third round.

It's the second significant injury news in two days for tennis. On Wednesday, 14-time major champion Rafael Nadal announced he hurt his right wrist in practice and would miss hard-court events in Toronto and Cincinnati, although he holds out hope of defending his title at the US Open.