Early Wednesday, Li Na came back from a set down and upset Williams, 2-6, 7-6 (7-4), 7-5. Li's ability to run down shots and her pinpoint placement proved to be too much for Williams.
Nadal, the defending champion, retired from Tuesday's quarterfinal against Andy Murray with an injured knee, then predicted the Scottish player would end his major singles title drought by winning the Australian Open.
"For Andy, I think he deserves to win his first Grand Slam [tournament]. And I think he's going to do it," Nadal said after a right knee injury forced him to concede the matchwhile trailing 6-3, 7-6 (2), 3-0. "There's a very good chance for him. First thing, he's playing very well. Second thing, he's already in the semifinals. He's only two matches away."
Nadal said he didn't want to risk more damage by playing and potentially having to spend long periods off the tour with knee tendinitis.
"Similar thing that I had last year," Nadal said of the pain. "It was impossible to win the match."
Roddick struggled with a shoulder injury before going down in five sets to Marin Cilic.
Murray will play a semifinal against Cilic, who beat No. 7 Roddick, 7-6 (4), 6-3, 3-6, 2-6, 6-3.