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Sloane Stephens of the US after her quarterfinal win over compatriot Serena Williams at the Australian Open tennis championship in Melbourne, Australia, Wednesday, Jan. 23, 2013. (AP Photo/Mark Baker)

Serena Williams' dominating run at the majors is over.

American teenager Sloane Stephens is headed to the semifinals of the Australian Open in Melbourne.

Williams hurt her back in the eighth game of the second set, slowing down her serve, restricting her movement and causing her obvious pain.

But the 19-year-old Stephens kept her composure, blocking out the injury issue on the opposite side of the net, and rallied for a 3-6, 7-5, 6-4 victory on Wednesday -- by far the most significant in her seven Grand Slams.

It was Williams' first loss since Aug. 17 for the 15-time Grand Slam winner, ending a run of 20 consecutive wins.

She hadn't lost a match at a Grand Slam tournament since the French Open, where her first-round exit sparked a resurgence in the second half of 2012 that included titles at Wimbledon, the London Olympics, the U.S. Open and the WTA Championship.

After winning her first Grand Slam quarterfinal, Stephens next plays defending champion Victoria Azarenka.

Williams' latest winning streak included a straight-sets win over Stephens at the Brisbane International earlier this month.

And Stephens wasn't even sure that she could beat Williams, until she woke up Wednesday.

"When I got up, I was like, 'Look, Dude, like, you can do this.' Like, 'Go out and play and do your best,' " she said.

It wasn't until after losing the first set and being broken in the first game of the second that she really convinced herself she could.

"I was like, 'Hmm, this is not the way you want it to happen. But you just fight and just get every ball back, run every ball down, and just get a lot of balls in play, I think you'll be OK.'

"From then on I got aggressive, started coming to the net more, and just got a lot more comfortable."

Williams walked around the net to congratulate Stephens, who then clapped her hand on her racket and waved to the crowd, a look of disbelief on her face.

She then went to her tennis bag, pulled out her phone and started checking for any text messages from her mother.

"I was hoping she had texted me right away. I thought maybe she was texting me during the match," Stephens said. "I'm sure my grandparents are like freaking out."

Stephens has said she had a photo of Williams up in her room when she was a child and had long admired the Williams sisters.

Williams was up a set and a break before Stephens settled in. In the eighth game of the second set, Williams was chasing a drop shot to the net when she appeared to hurt her back. She needed a medical timeout after the set, and then slowly started to regain the speed in her serve.

But she was clearly frustrated at times, smashing her racket into the court in the third set.

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  • John Isner and Sam Querrey will join Bob and Mike Bryan on the U.S. team to play Brazil in the first round of Davis Cup World Group matches from Feb. 1-3.

  • Spain will open the Davis Cup against Canada without injured star Rafael Nadal or regular member David Ferrer.