Williams upset in Melbourne Open

What was supposed to be a learning experience against one of the greatest tennis players in history turned into one of the biggest surprises in tennis history today, when 19-year-old Sloane Stephens introduced herself to a global audience by rallying to defeat her 31-year-old US elder Serena Williams 3-6, 7-5, 6-4.

Williams is a 15-time Grand Slam singles champion and was the No. 3 seed and heavy favorite here, but what made the result all the more unexpected was that she has been as dominant of late as she has been in the past: sweeping to the Wimbledon, Olympic and US Open titles last year and winning 20 straight matches coming into this quarterfinal.

But the streak and Williams' newfound tranquillity on court came crashing to a halt on this cool, sunlit afternoon in Rod Laver Arena as Williams, limited and frustrated by a back problem and Stephens' precocious blend of offense and defence, smashed her racket to smithereens early in the third set.

As a result, there will be no rematch between Williams, seeded third, and world No. 1 Victoria Azarenka in the semifinals Thursday. Instead it will be Azarenka versus Stephens, seeded 29th, who had never been past the fourth round in a Grand Slam tournament until this trip to Australia.

Although she has had other tennis role models besides Williams, including Kim Clijsters, Stephens once had a poster of Williams on her wall. "This is so crazy, but oh my goodness," Stephens said, wiping away tears in her post-match interview. "I think I'll put a poster of myself now."

It was the second huge grand slam shock in the past year for Williams, who was beaten in the first round of the French Open last year by the unseeded Frenchwoman Virginie Razzano.

But this was the first time that Williams, the best player of her generation, had been beaten by a younger American in tournament play.

Stephens said she felt good about her chances before the match began.

"Last night I was thinking about it," she said. "And someone asked me, 'Do you think you can win?' And I was like, 'Yeah, I think so.' But I wasn't too clear about it, and this morning when I got up I was like, 'Dude, you can do this. Go out and play and do your best."'

Williams and Stephens met only recently but they have had considerable contact in the past year. They were Fed Cup teammates last year and have spent time together in Los Angeles, where Stephens lives with her mother and younger brother and where Williams has a residence.

But they will now be rivals as well as teammates, and this defeat came less than a month after they played for the first time. Williams won that match in the quarterfinals in Brisbane in straight sets, but Stephens was surprisingly comfortable playing at Williams' torrid baseline pace, drawing big praise from Williams afterward.