Sloane Stephens stunned childhood idol Serena Williams to reach the Australian Open semi-finals today.

Williams' chances were seriously hindered by a back injury suffered midway through the second set but credit Stephens, who kept her nerve to complete a 3-6 7-5 6-4 win.

Stephens had a picture of Williams on her bedroom wall growing up and she joked in the post-match interview: "I will have to put up a poster of myself now!"

"I am in the semis of a grand slam so I will give it my all" - Sloane Stephens

Third seed Williams looked set to make routine progress through to the last four when she led by a set and a break, but Stephens had other ideas.

Starting to display the kind of form which carried her to her maiden grand slam quarter-final, she was hitting freely from the back of the court and made several moves forward to finish points off at the net.

It was turning into a real contest between the experienced Williams and the 19-year-old, understandably tipped as her heiress apparent in American tennis.

But then the match took another turn as Williams sustained a back spasm in lunging for a low ball at the net.

The teenager broke for a 5-3 second-set lead and although she could not immediately serve it out - Williams, after taking a medical time-out, bravely hitting back to level it at 5-5 - Stephens regrouped following a bout of jitters to break again and take it to a decider.

Williams' frustration boiled over after three games of the final set, earning herself a code violation for whacking her racket into the court.

But she could console herself with the fact her back appeared to be loosening up, perhaps as a result of the medication prescribed earlier starting to kick in.

Williams broke for a 4-3 lead but Stephens was not going away and she hit straight back before edging 5-4 up.

And this time it was Williams who buckled as a series of tired groundstrokes gifted Stephens the win of her life.

She will now meet world number one Victoria Azarenka, who earlier beat Svetlana Kuznetsova 7-5 6-1.

"It should be fun," said Stephens. "I am in the semis of a grand slam so I will give it my all and do the same thing again."

Azarenka saw off a spirited display from Kuznetsova to keep the defence of her title on track.

Kuznetsova, a two-time former grand slam champion, proved she is over the knee surgery which forced her to miss the second half of 2012 with a wonderfully creative display - mixing clever angles with raw power.

But it was not enough to derail Azarenka, who, having edged the first set, ran through the second to register her 12th successive win in Melbourne.

"She played really well throughout the whole match," said Azarenka. "I know what kind of tennis she is capable of and in the beginning it took a little bit of adjustment because she has a very different game.

"I am glad I could produce my best tennis when it was needed."
Kuznetsova led 4-1 in the first set after displaying a brand of tennis which bamboozled the top seed.

But Azarenka eventually worked it out and powered her way back into the contest.

She broke back for 3-4 and then ramped up her performance level even further at 5-5 with a big forehand on break point edging her ahead for the first time in the match.

She served it out and then maintained the momentum by breaking in the opening game of the second.

Kuznetsova hit back immediately but she could not win another game as Azarenka marched on.