MELBOURNE, Australia (AP) -- Serena Williams was only thinking out loud when she muttered that this Australian Open had been "the worst two weeks."
Not long after a courtside microphone picked up those comments during her quarterfinal with 19-year-old American Sloane Stephens, things got a whole lot worse.
Stephens outplayed Williams, whose movement and serves had been slowed by a back injury, and beat the 15-time Grand Slam champion 3-6, 7-5, 6-4. It was Williams' first loss since Aug. 17, and her first defeat at a Grand Slam tournament since last year's French Open.
Williams' downer of a Grand Slam Down Under started badly when she turned her right ankle in her opening match at Melbourne Park.
"I've had a tough two weeks between the ankle ... and my back, which started hurting," Williams said. "A lot of stuff."
While Williams packed for home -- she and sister Venus have also lost in doubles -- Stephens advanced to a semifinal Thursday against defending champion Victoria Azarenka.
The top-seeded Azarenka beat Svetlana Kuznetsova 7-5, 6-1 in the early quarterfinal at Rod Laver Arena. Maria Sharapova, who has lost only nine games in five matches, plays Li Na in the other semifinal Thursday.
On the men's side, Andy Murray advanced to the semis with a 6-4, 6-1, 6-2 victory over Jeremy Chardy. Murray, the U.S. Open champion, will play the winner of Wednesday's late quarterfinal between No. 2 Roger Federer and No. 7 Jo-Wilfried Tsonga.
The other semifinal has defending champion Novak Djokovic taking on No. David Ferrer on Thursday.
Williams hurt her back in the eighth game of the second set and things got progressively worse. She yelled at herself on several occasions, and smashed a racket into the court, earning a $1,500 fine from tournament officials.
"I was running to the net for a drop shot," Williams said, describing the lead up to her injury. "As I went to hit it, it was on the backhand. I even screamed on the court. I totally locked up after that."
She reiterated after the match that her injuries had made this Australian Open difficult for her.
"Absolutely, I'm almost relieved that it's over because there's only so much I felt I could do," she said. "I've been thrown a lot of (curve) balls these two weeks."
Stephens has been, too, but has coped well, and the magnitude of her accomplishment only hit her while she was warming down after the match.
"I was stretching, and I was like, 'I'm in the semis of a Grand Slam.' I was like, 'Whoa. It wasn't as hard as I thought,'" she said. "To be in the semis of a Grand Slam is definitely a good accomplishment. A lot of hard work."
The No. 29-seeded Stephens hadn't been given much of a chance of beating Williams, who lost only four matches in 2012 and was in contention to regain the No. 1 ranking at the age of 31.
Williams' latest winning streak included a straight-set win over Stephens at the Brisbane International this month.
Stephens wasn't even sure that she could beat Williams until she woke up Wednesday.
"When I got up, I was like, 'Look, Dude, like, you can do this.' Like, 'Go out and play and do your best," she said.
Williams walked around the net to congratulate Stephens, who then clapped her hand on her racket and waved to the crowd, a look of disbelief on her face.