Sloane Stephens upsets Serena, while Federer advances at Australian Open

Serena Williams went down smashing rackets and screaming as she bowed out of the Australian Open quarterfinals on Wednesday, hampered by a back injury and beaten in three sets by fellow American Sloane Stephens.

The injury robbed Williams of her serve - the most effective weapon in women's tennis - but teenager Stephens will take much credit for holding her nerve to finish off the ailing 15-time grand-slam champion.

Roger Federer's bid to emulate Williams as a five-times Melbourne Park champion survived a five-set test at the hands of an inspired Jo-Wilfried Tsonga and the Swiss marched on to a last-four meeting with Andy Murray, who crushed Jeremy Chardy.

Stephens will have 24 hours to prepare for her first grand-slam semi-final against defending champion and world number one Victoria Azarenka, who came through a minor scare to beat Russian Svetlana Kuznetsova.

"Oh my goodness," said Stephens, teary-eyed and almost lost for words after beating a player whose picture once adorned her bedroom wall. "This is so crazy, but oh my goodness, I think I'll put a poster of myself up now."

The 31-year-old Williams, odds-on favorite to claim a third successive grand slam crown, pulled up to avoid hitting the net after a backhand drop shot early in the second set and shrieked as she felt the full force of a back spasm.

After lengthy treatment, Williams continued but the power of her serve and ground strokes were considerably diminished and 19-year-old Stephens took advantage in impressive fashion to run out a 3-6 7-5 6-4 winner in Rod Laver Arena.

Williams, who smashed her racket to pieces in frustration in the third set, tried graciously to honor the locker-room code that you do not diminish an opponent's achievement but was unable to wholly play down the significance of the injury.

"I even screamed on the court," she said. "I was like, 'ahh'. I totally locked up after that. It was...a little painful.

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