In the course of 20 minutes, her impressively wide yawns during stretching exercises gave way to barely under-the-breath commentary when her groundstrokes weren't firing.
With Serena's parents, Richard and Oracene, watching from opposite sides of the court, her hitting partner of three years, Aleksander "Sasha" Bajin, began to gain the upper hand. When she sent an unconvincing forehand into the net, Serena flung her Wilson racket, which skipped over the red clay and came to rest about 20 feet away.
The French Open has always been the most difficult of the Grand Slam events for Serena; the cloying clay is not conducive to her power game. She knows it, and so do her opponents.
Stosur, a 26-year-old from Brisbane, Australia, moving like a cat on the clay at Roland Garros, mixing artful backhand slices with deep, penetrating forehands and a feisty kick serve, was throttling the world No. 1 for a set and eight games Wednesday.
But then, serving for the match at 5-3, Stosur (almost predictably) shrank from the greatest moment of her tennis life when it first presented itself. She threw in her first double fault and made a truly egregious shot choice as Serena broke her for the first time. The tiebreaker was a foregone conclusion. And so, apparently, was the match.
"After losing that game, yeah, things kind of stepped up a notch from her," Stosur said after the match. "Yeah, I didn't know if I was going to get another chance.
"That third set just kind of hung in there, and I was behind the whole time serving second. Just tried to hang in and wait for another opportunity. When I got that next chance at 7-6, I wasn't going to let the same thing happen again."
Somehow, Stosur weathered Serena's (also predictable) charge, mastering her frayed nerves, saving a match point in the 10th game and saving her best tennis for the end. Stosur broke Serena with two ludicrous passing shots -- a fiery cross-court forehand and a lunging backhand -- and prevailed 6-2, 6-7 (2), 8-6.
Stosur won more games (plus-five) and points (12) than Serena, played more efficiently and, at the end, less emotionally in a 2-hour, 24-minute match. Her net of winners to unforced errors was plus-6, Serena's minus-7.
On several occasions, Serena was given the opportunity to give Stosur credit for playing well. She generally declined the invitation.
"It was my match -- and I lost it. That's what happened," said Serena, who appeared for her interview several hours after the match following a doubles victory with sister Venus. "I'm a little disappointed. I was definitely off.
"I was more or less upset that I was able to play so well up to this point. I couldn't believe, when it was over, that I played like that. If I played better for maybe two minutes, the result might have been different."
This was Serena's 10th French Open and, with the exception of her lone win in 2002, someone has always sent her home early. It was Arantxa Sanchez-Vicario in Serena's first appearance a dozen years ago and, over time, Jennifer Capriati and Justine Henin (twice each) and Svetlana Kuznetsova also have taken her down in Paris.
Coming into this French Open, four-time champion Henin was widely viewed as the favorite, but it is Stosur who has played the best clay-court tennis among women. She's now 19-2 on the dirt, and when she beat Henin in the fourth round, she became the favorite in some eyes -- if she could lock down her emotions the way she did against Henin.
The two players settled into the first set, trading some massive groundstrokes. But with Serena serving at 2-3, Stosur introduced the slice -- and it bothered Serena. Three consecutive backhand slices led to an errant forehand by Serena, giving Stosur her first break-point conversion.
When Serena was broken at love to end the first set, an uneasy buzz traveled through the crowd at Court Philippe Chatrier.
Stosur had lost three of four previous meetings with Serena (they had never played on clay), and their 2009 match in Sydney underlined the dynamic of their relationship. Stosur had four match points in that contest but could not close the deal.
After failing in the second set, Stosur managed to rally in the third when it looked as if Serena had regained the momentum of the match. Going forward, it should be a valuable resource for her.
"It's a bit different from playing Justine and Serena," Stosur said Wednesday. "I know she's going to hit tons and tons of balls in. I have to be prepared to be patient and wait for my opportunity a bit more.
"I think I've got as good a chance as anyone. I'm in the semis now; I played two great matches. Hopefully, they're both going to help me for tomorrow's match."
Greg Garber is a senior writer for ESPN.com.