Melbourne, Australia -- There's been a stunning upset at the Australian Open. Serena Williams is out after losing her quarterfinal match to 19-year-old American Sloane Stephens 3-6, 7-5, 6-4.
It was the first loss since Aug. 17 for the 15-time Grand Slam winner, ending a run of 20 consecutive wins.
Williams appeared to hurt her lower back while attempting to pull up short of the net during the eighth game of the second set. She called for a trainer after the ninth game, but said the injury was "no excuse" for the loss.
Next up for Stephens is defending champion Victoria Azarenka. She advanced to the semifinals with a 7-5, 6-1 win over former two-time Grand Slam winner Svetlana Kuznetsova.
In the men's draw, U.S. Open champ Andy Murray reached the semifinals with a 6-4, 6-1, 6-2 win over unseeded Frenchman Jeremy Chardy.
Murray, who ended a 76-year winless streak by British men in majors by winning the US Open last year, will play either Roger Federer or Jo-Wilfried Tsonga in a semifinal on Friday.
Defending champion Novak Djokovic plays No. 4-seeded David Ferrer in the other semifinal.