Williams served for the match in the second set against Li but was broken, forcing the set to the tiebreaker which Li dominated. Williams held service in the first game of the third set and Li in the eighth, but in between there were six consecutive breaks of serve — and eight in the first 10.
Li broke Williams' serve again in the 11th game of the decider, then held to win on her third match point in 2 hours, 47 minutes. Li made 57 unforced errors in the match, Williams 53.
"Best day of my whole life," Li said on court. "I just played my tennis. I know I lost the first set but thanks for the coach, never give up."
Li and Zheng Jie have became the first Chinese players to advance to the semifinals of the same Grand Slam tournament. Zheng plays Justine Henin on Thursday.
"Good for both players, also good for China's tennis," Li said.