BEIJING—Li Na, Asia's biggest tennis star, said Friday she is retiring from the sport, capping a career in which she became the first Asian woman to win a Grand Slam, as well as an iconoclastic personality that made her one of China's most marketable sports stars.

The 32-year-old said in statements via her verified Chinese social-media microblog and Facebook that four surgeries on her knees to heal pain and swelling had taken their toll, including the most recent in July on her left knee. "My chronic injuries will never...