China's Li Na, Asia's only grand slam singles champion, announced her retirement from tennis, succumbing to the effect of long-term knee injuries. A former French Open winner and the reigning Australian Open champion, Li said her troublesome knees, that have plagued her throughout her career, prevented her from ever regaining full fitness and forced her hand. "The task of finally making a decision to hang up my racquet felt a lot more difficult than winning seven matches in a row in the Australian heat," she said in a letter posted on her Facebook page. "It took me several agonizing months to finally come to the decision that my chronic injuries will never again let me be the tennis player that I can be. Walking away from the sport, effective immediately, is the right decision for me and my family." The 32-year-old, known affectionately as "Big Sister Na" and "Golden Flower" in China, won the 2011 French Open and this year's Australian Open. She climbed to number two in the world rankings but her knees were steadily getting worse and she skipped this year's US. Open, triggering speculation her dazzling career was drawing to a close.
