MELBOURNE, Australia — Serena Williams was only thinking out loud when she muttered 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 marked Williams’ first loss since Aug. 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 sideline. But Federer 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 match points in the previous game. Federer, who advanced to the semifinals for the 10th consecutive year at Melbourne Park, will play U.S. Open champion Andy Murray on Friday.