JOIN USWozniacki, seeking a first Grand Slam title to back up her world number one ranking, survived a hiccup while serving for the match to defeat determined German Andrea Petkovic 6-1, 7-6 (7/5) yesterday.
Williams, whose 13 Grand Slam titles include three US Open crowns, shrugged off a shaky start to defeat 20-year-old Anastasia Pavlyuchenkova 7-5, 6-1.
Stosur extended her dominance over Vera Zvonareva as she overpowered the world number two 6-3, 6-3, and Kerber, ranked 92 in the world, and playing in her first Grand Slam quarter-final, ousted experienced Italian Flavia Pennetta 6-4, 4-6, 6-3.
While Wozniacki is the top seed and Williams a lowly 28th after her lengthy layoff due to injury and illness, the American has stamped herself the favorite by reaching the final four without dropping a set.
She warmed-up for the Open with hardcourt titles in Stanford and Toronto, and the weather delays that have wreaked havoc on the tournament have hardly touched her.
"I was fortunate to have the night match so I came out when I was supposed to, and then got cancelled shortly after," Williams said of her abbreviated appearance on court with Pavlyuchenko on Wednesday night.
They didn't get past their warm-up before the night session was called off as rain wiped out play for the second straight day.
"It wasn't too much of a burden. The day before was my day off, so I actually had probably a good schedule," Williams said.
Even so, she got off to a slow start against Pavlyuchenkova in a match that started with a rash of service breaks before Williams found her groove.
"She's definitely different," Williams said of Pavlyuchenkova. "She'll hit hard, then she'll hit soft. She's great at mixing things up.
"I just think I was a step slow in the beginning. I was hitting a lot indoors and it's a totally different surface.
"Once I kind of got out in the match, once I showed up, I was able to play better."
Wozniacki had some adjusting to do when her match was moved to court 13 from water-logged Louis Armstrong Stadium, where the legacy of days of heavy rain was water seeping up through the court surface behind one baseline.
"It was much different," Wozniacki said. "The court was a bit faster. I felt the ball was not jumping as high. And you feel a bit different because the crowd is sitting closer.
"But it's the quarter-finals of a Slam so you just go out there ... You just have to play inside the lines."
Wozniacki served for the match at 5-3 in the second but dropped that service game and her next as Petkovic forced the tiebreaker.
Petkovic saved two match points in the decider with volley winners, but a backhand long meant the 11th-ranked German didn't get another chance to do her body-popping celebration dance.
"It was just a couple of balls that made the difference, and unfortunately it didn't go my way," Petkovic said.
Stosur may have actually benefitted from the two days of enforced rest as she reached the US Open semis for the first time.
After a pair of marathon victories in the third and fourth rounds, she needed just 67 minutes to notch her eighth straight victory over Zvonareva.
"It's always nicer to finish them off in less than three hours," said Stosur, who took a US Open women's record three hours and 16 minutes to beat Nadia Petrova in the third round.
In the fourth round she rebounded to beat Maria Kirilenko after dropping a 17/15 second-set tiebreaker.
"To come out and play the way I did today after the long waiting around that we have had to do is really, really pleasing," added Stosur, who won 12 straight points to finish off the opening set and start the second, fired 25 winners and never faced a break point.
"She was able to hold her serve pretty easy, that's why she could take a lot of risks on mine," said Zvonareva, who has yet to return to a Grand Slam final after runner-up finishes at Wimbledon and Flushing Meadows last year.
"She was over-playing me a little in every game."