Toronto, Canada – By Ian Ransom
The 16th seed's upset victory in Melbourne also guaranteed China will have two players in the last four of a major for the first time.
In a match dotted with service breaks, Li held her nerve at 6-5 in a pulsating final set, sealing a famous win with a cracking forehand winner on her third match point.
"The best day of my whole life," Li said in a courtside interview. "I just played my tennis ... (it's) good for tennis (in) China and good for the fans."
Williams, whose form in Melbourne has been perennially hot and cold, was again erratic against Li, steam-rolling the Chinese in the first set before losing her way in the second.
Handed a golden opportunity to seal the match early, the sixth-ranked American froze when serving at 5-4, spraying four unforced errors to allow Li to break back and take the set in a tie-breaker.
Li, who has a reputation as a scrapper, pounced on the chance, smacking a forehand winner to bring up two set points and forcing an error to take the tiebreaker 7-4.
Williams had the chance to seize the initiative after breaking Li immediately to take a 2-0 lead in the third, but was again found wanting.
The hard-hitting Chinese promptly broke back to kick off an uninterrupted chain of six breaks of serve but then caught the wobbles herself when serving for the match at 5-4.
Williams clawed her way back to 5-5 but surrendered her serve again, opening the door for Li to slip into the semi-finals and send Chinese fans in the Rod Laver Arena into ecstasy.
(Editing by Alastair Himmer)