"In the semi-finals, you've got to believe anything is possible," Stosur said Friday after moving into the semis with a 63-minute 6-4, 6-1 win over unheralded Roberta Vinci of Italy.
"Having said that, from the semi-finals to winning the tournament, there's still a long way to go," Stosur added cautiously. "I think in this position you've got to (think you can win). If you don't, then it's probably a little bit silly."
After a day full of upsets that forced many of the biggest names in tennis to make an early exit, sanity was finally restored at the Rexall Centre.
The top two remaining seeds, Stosur and No. 4 Victoria Azarenka of Belarus, advanced to Saturday's semis with one-sided wins while Radwanska moved on with a minor upset.
Williams, unseeded and ranked 80th after taking almost a year off due to injuries, needed three sets to shrug off 32-ranked lefty Lucie Safarova of the Czech Republic 4-6, 6-3, 6-2 and book her berth in the semis against Azarenka.
"She's the better-ranked player and has had a fantastic year," Williams said of Azaenka. "She's a power player. She's smart, she's young and she has fresh legs. I just have to go in there and try to do my best."
A day after the carnage that left fans scrambling for their programs, reality set in again when Azarenka over-powered qualifier Galina Voskoboeva 6-1, 6-2 to set up a quarter-final showdown with Williams.
The 22-year-old Russia, who has lost just six games in her quiet run to the final four, is the highest seed left in the tournament.
"(I'm) not really focused on that, honestly," Azarenka said. "I'm just here to play the tournament, (I'm) not looking at the rankings or anything else. (I'm) just trying to stay focused on every game. Every opponent is tough, as you can see throughout the tournament, so I'm just really trying to be focused on myself.
Voskoboeva, who needed to win a pair of weekend matches just to get into the main draw, gained some admirers with a gutsy run that saw her beat No. 9 Marion Bartoli and fan favourite Maria Sharapova, the fifth seed. But she had no answer for Azarenka less than 24 hours after showing Sharapova the door.
"It's one week," Azarenka said of Voskoboeva. "But if she can do that on a consistent basis, she will be up (in the) top 10."
Regardless of the loss, Voskoboeva, ranked 135 in the world, called it the best week of her life.
"She's a great player and you can't wait for her mistakes, she doesn't make them," Voskoboeva said. "(Thursday) I played better than today."
Stosur had no trouble with Vinci and will face Radwanska, a 6-4, 6-3 winner over Andrea Petkovic of Germany in the semis.
Stosur, No. 11 in the world, is 1-1 in two previous meetings against Radwanska, the 13th seed here but ranked one spot behind the Aussie. The two are so even across the board they might as well be looking in the mirror.
"For sure, you know, she's a great player, great champion," Radwanska said, sounding a little like a Valley Girl with a Polish accent. "We played a couple of times before but I don't know the statistics, I can't remember. I have to play my best tennis to beat her. It's going to be a completely different match than (Friday). But, you know, I'll try."
Petkovic, who has bewitched many of the media covering this tournament with her engaging personality, is still looking for her first win in four meetings against Radwanska.
The 22-year-old from Krakow beat Petkovic in the semifinals of the Mercury Insurance Open in Carlsbad, Calif., en route to the title last weekend.
Maybe knowing she was 0-3 going into Friday's match caused Petkovic to change her strategy. The move backfired.
"I felt like, I don't know, I needed to overpower her, which was the wrong approach," Petkovic said. "I think there were five or six important points where I missed it this much. So, yeah, it's a little bit of bad luck and maybe I should have been a little tougher in the long rallies but she played well."