MELBOURNE - A day after earning the biggest win of her career, American teenager Sloane Stephens faces another massive challenge when she meets defending champion Victoria Azarenka in the Australian Open semifinals on Thursday.

The 19-year-old stunned the tennis world on Wednesday when she beat 15-times grand slam champion Serena Williams 3-6 7-5 6-4 in the quarter-finals at Melbourne Park.

The victory was the biggest upset of this year's tournament, where the top seeds in both the men's and women's draws have found their progress relatively unimpeded.

Sloane Stephens of the US celebrates defeating compatriot Serena Williams during their women's singles quarterfinal match at the Australian Open tennis tournament in Melbourne, Jan 23, 2013. [Photo/Agencies]

"I'm in the semis of a grand slam," said the 29th-seeded Stephens after beating her idol. "I was like, 'Whoa. It wasn't as hard as I thought'. But it's pretty cool.

"To be in the semis of a grand slam is definitely a good accomplishment. It's nice to be in this position."

Azarenka, who dispatched Svetlana Kuznetsova earlier on Wednesday to reach the last four, had described Sloane as "a very, very talented girl" before the match with Williams.

"I think over the last couple years you see her development as a tennis player. Her game has come together," added the defending champion.

With Williams now out of the picture, Maria Sharapova will feel her chances of clinching a second Australian Open title have increased dramatically, but she must first overcome a more focused and disciplined Li Na, whose fortunes have been turned around after Argentine Carlos Rodriguez took over as her coach.

"She certainly stepped up her game," Sharapova said of the Chinese former French Open winner.

"I think at different times in a player's career you need that extra motivation, you might need a change.

"It wasn't like she needed someone to come in and fix her game. But sometimes when you just have ... a new voice, they might be saying the same things but it just gets to you a little bit differently and your motivation changes."

Men's champion Novak Djokovic will conclude the singles action on Thursday when he meets fourth seed David Ferrer in the semifinals and the Serb is well aware that he cannot sit back against one of the fittest players on the ATP Tour.

"I need to be aggressive on the court, that's for sure," Djokovic said. "I need to step in and try to be in control of the match, otherwise he makes his own rhythm, he makes his own pace on the court. That's where he's very dangerous.

"He's somebody that has a lot of respect from all the players because he's playing so many tournaments and works very, very hard.

"You can see because he's ... one of the fittest players around and is playing the best tennis of his life in the last 15 months.

"It's the semis of a Grand Slam, so I expect a tough match."