The makeup of the men's semifinals was as expected, although all but one of the four players had some difficulties getting there.
Top-ranked Novak Djokovic will continue his bid for a third consecutive Australian title Thursday when he takes on No. 4 David Ferrer. No. 2 Roger Federer and No. 3 Andy Murray will meet Friday.
Djokovic had to get through a five-hour five-setter against No. 15 Stanislas Wawrinka in the fourth round. Ferrer had trouble in his quarterfinal against fellow Spaniard Nicolas Almagro, who served for the match in the third and fourth sets before losing in five.
Federer, a 17-time Grand Slam champion, hadn't dropped serve in the tournament until the first set against 2008 finalist Jo-Wilfried Tsonga on Wednesday. He ended up with a struggle on his hands before advancing to his 10th consecutive Australian Open semifinal with a 7-6 (4), 4-6, 7-6 (4), 3-6, 6-3 win in 3 hours, 34 minutes.
"I thought he played very aggressive," Federer said. "I love those four-set or five-set thrillers, and I was part of one tonight."
Murray beat Jeremy Chardy of France 6-4, 6-1, 6-2 earlier Wednesday and hasn't dropped a set this tournament.
Stephens' previous best run at a major tournament was at last year's French Open, where she was the first American teenager to reach the fourth round since Williams in 2001.
She was the youngest player in the year-end top 50 despite missing the last six weeks of the season with an abdominal injury.
"I took a lot of time off just kind of being a normal kid, doing whatever," she said.
That means shopping, social networking and other things teens do.
Stephens was under real pressure early, and it showed. She double-faulted to give Williams triple break point in the eighth game, but could save only two. Williams started pumping her fist and yelling, "Come On."
Williams broke her serve again in the opening game of the second set.
"From then on, I got aggressive, started coming to the net more and just got a lot more comfortable," Stephens said.
That paid off when she broke back in the fourth game.
Williams had injured her ankle in her first-round win, and the back injury compounded her problems in what she said later was her worst major tournament in a long time.
The five-time Australian champion shanked a forehand to fall behind 2-1 in the third, then smashed her racket into the court, twice, flinging it toward the courtside chairs.
She picked up her service speed and was called for a foot fault, further annoying her.
Williams got the first break of the set and seemed to be back on track for victory, but Stephens answered immediately.
Serving to stay in the match, Williams hit an attempted passing shot long and looked up to the sky, muttering to herself.
She hit backhands into the net on the next two points, and her winning streak had ended.
Williams walked around the net post to shake Stephens' hand. The quick turnaround between matches makes it difficult for Stephens' family to make it to Australia for her semifinal.
"I'm kind of upset my mom's not here, and my brother," she said. "I know definitely everyone's watching back home and is very proud of me, so (I'll) just do my best and make them happy."