BEIJING (AP) — Relying on fitness and a strong baseline game, Li Na reached the pinnacle of tennis and lifted the level of the sport in Asia to unprecedented heights. A sense of humor along the way certainly helped. The legs that carried Li to two Grand Slam titles ultimately couldn't get her through another season, with the 32-year-old Chinese tennis star citing recurring knee injuries as the reason she needed to quit when she announced her retirement in an open letter posted to social networks on Friday. It ended weeks of speculation and hype on the Chinese social networking sites about Li's career coming to an end. The WTA, which governs women's tennis, has described Li as a trailblazer after becoming the first player from Asia to win a major title — the 2011 French Open, beating four top 10 players in succession to wrap up the title — a few months after becoming the first from the region to reach a Grand Slam final, at the 2011 Australian Open. In her third trip to the final at Melbourne Park, Li won the Australian Open title in January to reach a career-high No.