The only tennis player from China to win a Grand Slam singles title also withdrew from next month's hard-court tournament in Cincinnati.
"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 US Open," she said in a statement released by Tennis Canada. "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 heals.
"I look forward to getting back on the court this fall in Wuhan and Beijing."
Li played in the last four editions of the Rogers Cup. In 2012, she reached the final in Montreal before falling to Petra Kvitova of the Czech Republic.
As a result of the withdrawal, Eugenie Bouchard of Westmount, Que., becomes the fifth seed at the Aug. 1-10 tournament at Uniprix Stadium. She'll get a first-round bye along with the other top eight seeds.
Also Thursday, tournament organizers announced that Yvonne Meusburger of Austria will miss the event and American players Barbora Zahlavova Strycova and Alison Riske will enter the main draw.
The official draw will be held Friday and the qualification rounds begin Saturday.
Li did not specify which knee was injured, but she played with her right knee taped up at Wimbledon, her most recent tournament. The U.S. Open begins 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.
On Wednesday, 14-time major champion Rafael Nadal announced he hurt his right wrist in practice and would miss the Rogers Cup in Toronto and the Cincinnati event, although he holds out hope of defending his title at the U.S. Open.
With files from The Associated Press.