(CNN) -- American teenager Sloane Stephens stunned her compatriot Serena Williams on Wednesday, beating the 15-time grand slam winner 3-6 7-5 6-4 to reach the Australian Open semifinals.
The 29th seed, playing in her first grand slam quarterfinal, came from once set down to overcome third seed Williams who suffered a back injury having already rolled an ankle earlier in the tournament.
Despite Williams' fitness problems, few had given Stephens a chance of beating the five-time Melbourne champion.
"Someone asked me, 'Do you think you can win?' I was like, 'Yeah, I think so, but I wasn't like too clear about it,''" the 19-year-old told reporters after advancing to a tie with world No. 1 Victoria Azarenka on Thursday.
"Then this morning when I got up, I was like, 'Look, dude, like, you can do this, go out and play and do your best.'"
Stephens did not make the best possible start as Williams went one set and a break of serve ahead, before the 19-year-old American dug deep to pull out an unlikely triumph.
"You just fight and just get every ball back, run every ball down, and just get a lot of balls in play," said Stephens, explaining her strategy after the loss of that first set.
"From then on I got aggressive, started coming to the net more, and just got a lot more comfortable."
Stephens becomes the first U.S. teenager to reach a grand slam semfinal since Serena at the 2001 U.S. Open, and the first to achieve the feat outside of the Williams sisters since Alexandra Stevenson at Wimbledon in 1999.
For Williams, it marked the end of a campaign which had pushed her body to the limit.
"I'm almost relieved that it's over because there's only so much I felt I could do," said the 31-year-old. "It's been a little difficult. I've been thrown a lot of balls these two weeks."
When one journalist asked Williams if she had considered retiring hurt during the match, it caused some confusion in the press room.
"Are you kidding me? I'm not retiring," responded Williams, believing the reporter was asking if she was considering ending her career.
"Oh, you mean, retiring in the match! Oh, I'm sorry. I thought you meant my career. You're crazy.
"I thought about it like for a nanosecond. It's a quarterfinal of a grand slam. Even if I have to take off in a wheeler before I retire."
Defending champion Azarenka had to battle through a tough 77-minute first set against Russian Svetlana Kuznetsova, before winning 7-5 6-1.
When asked about Stephens prior to her victory against Williams, Belarus' Azarenka said: "I think Sloane is a very, very talented girl. I think over the last couple years you see her development, you know, as a tennis player.
"Her game has come together. She's an all round player, very competitive. I think she has a good potential. It's just a matter of her getting all those details in place and really wanting to achieve great things."
The second semifinal, also due to take place on Thursday, sees sixth seed Li Na take on No. 2 seed Maria Sharapova -- who is yet to drop at set at the tournament.