MELBOURNE Jan 23 (Reuters) - Serena Williams went down fighting like the five-times champion she is but bowed out of the Australian Open quarter-finals on Wednesday, hampered by a back injury and stunned 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-times grand slam champion 3-6 7-5 6-4.
Stephens, one of a band of promising young American women, will next face defending champion and world number Victoria Azarenka, who earlier came through a first-set scare to beat experienced Russian Svetlana Kuznetsova 7-5 6-1.
"Oh my goodness," said Stephens, who was teary-eyed and lost for words after beating a player whose picture once adorned her bedroom wall to reach her first grand slam semi-final.
"This is so crazy, but oh my goodness, I think I'll have a poster of myself up now."
The 31-year-old Williams, odds-on favourite to claim a third successive grand slam crown, won the first set but pulled up to avoid hitting the net after a backhand drop shot early in the second and shrieked as she felt the full force of a back spasm.
After lengthy treatment, Williams continued but the power of her serve and groundstrokes were considerably diminished and her 19-year-old opponent took advantage in impressive fashion to run out a winner on Rod Laver Arena.
Williams tried graciously to honour the locker room code that you do not diminish an opponent's achievement but was unable to wholly disguise 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 just like, it was a little painful.
"I couldn't really rotate after that, which I guess is normal. I don't know.
"It was giving me trouble. But it was fine. I think my opponent played well and was able to do a really good job."
The match between Williams and Stephens had been variously billed as a clash between patron and protege, hero and hero-worshipper and even queen and pretender to the throne.
Whatever the truth about the nature of their relationship, Serena prevailed in a tight first set and looked in good shape to reach the last four until the injury intervened.
She smashed a racket to pieces in frustration in the third set as her body failed to respond to treatment and, facing two match points, could only find the net tape and her 50th grand slam campaign was over.
"This morning when I got up, I was like, 'Look, dude, you can do this'. Go out and play and do your best," Stephens said.
"(When she got injured) I really just wasn't thinking about anything other than myself because I was like, 'I'm going to get distracted. I'm going to start thinking about something else and it's going to be a mess.'"
Azarenka had barely been tested on her way to the last eight and the 77-minute first set against former U.S. Open and French Open champion Kuznetsova was longer than her entire fourth round match.
Kuznetsova, returning after missing the entire back end of last season with a knee injury, seemed to catch Azarenka by surprise and raced to a 4-1 lead in bright morning sunshine at Melbourne Park.
Azarenka rode the storm, however, and the slices with which the Russian had bamboozled the Belarussian grew steadily less effective as the world number one drove home her advantage.
"It was important to take the opportunities to kind of make a big statement," said Azarenka. "I think I did that and I turned things around into my own way."
Kuznetsova, whose slump to number 75 in the world began long before her knee injury, said she was still feeling the effects of her long lay off.
"These matches, you got to play very consistent," said the former world number two. "This is what for the moment kills me after my break, is my consistency. And I didn't serve well."
Britain's Andy Murray, the men's third seed, meets unseeded Frenchman Jeremy Chardy in the third match on Rod Laver Arena to decide who will play Roger Federer or Jo-Wilfried Tsonga in the semi-finals. (Editing by John O'Brien)