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 quarter-final 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 August 17, and her first defeat at a Grand Slam tournament since last year’s French Open.

Four-time Australian Open winner Roger Federer, a 17-time Grand Slam champion, looked for a while like he might join Williams on the sidelines. But he eked out a 7-6 (4), 4-6, 7-6 (4), 3-6, 6-3 win over 2008 finalist Jo-Wilfried Tsonga in a match that lasted 3 hours, 34 minutes.

Federer, who broke Tsonga in the fourth game of the deciding set, converted his fifth match point while serving after Tsonga saved four in the previous game. Federer, who advanced to the semi-finals for the 10th consecutive year at Melbourne Park, will play US Open champion Andy Murray tomorrow.

Murray advanced earlier yesterday with a 6-4, 6-1, 6-2 victory over Jeremy Chardy. The other men’s semi-final has defending champion Novak Djokovic playing David Ferrer today.

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 semi-final today against defending champion Victoria Azarenka.

The top-seeded Azarenka beat Svetlana Kuznetsova, 7-5, 6-1, in the early quarter-final at Rod Laver Arena. Maria Sharapova, who has lost only nine games in five matches, plays Li Na in the other semi-final.

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.

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 yesterday.

“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.