Samantha Stosur brought all the big weapons to the semifinal she played against Sara Errani at Roland Garros, including her big kick serve, atomic forehand, sculpted and toned body, and the signature one-two punch that's so useful against a woman whose serve would be hard-pressed to break the shell of an egg: Step around that serve to blast back a big forehand cross-court return, following with a putaway back the other way.

But Errani brought the biggest, most mysterious, and most valuable weapon of all: A strong mind. As a result, the 5'4" dynamo who's a mere No. 21 a seed in Paris advanced to the French Open final with a 7-5, 1-6, 6-3 triumph. Yet again, Stosur's inexplicable mental lapses and tendency to choke when put under pressure leaves me wondering how she ever managed to beat Serena Williams and win the U.S. Open last year.

Errani did many things right in this match, the first of which was her observance of what I call the "Janko (Tipsarevic) Law," which is the observation the Serb once offered that job No. 1 for an underdog is to stay even in any given set until those critical eight or ninth games because, as Janko put it, "Anyone, but anyone, can choke."

And that holds doubly for a player like Stosur, the No. 6 seed who's spent most of her career living with a reputation as a "head case," prone to running off the rails when the pressure is turned up. Errani managed to bring the issue into play when she recovered from a poor start (broken for 0-2) to break right back in the opening set. The women proceeded on serve long enough for Janko's Law to kick in at 5-5.

Serving the next game, Stosur's first serve vanished. She shanked a forehand and made two ugly errors interrupted only by a second-serve winner to Errani's backhand. But Stosur regained her composure to stave off two break points. Errani hung in there, though, and got to advantage again. She won the point to get the break when Stosur pulled a surprise attack and banged a backhand volley into the net. Errani served out the set without further incident.

In the second set, Errani experienced a mental glitch of her own—her only one of the match. Stosur held the first game, but at 0-1, 30-all, Errani hit a double fault and made a forehand error to yield the break. Curiously, the double fault was somewhat controversial, as Errani objected when Stosur hastily circled a mark that the chair umpire confirmed was out. Problem was, Errani said he was looking at the wrong mark, and the TV replay confirmed that the ball was in.

If that was the wrong mark, it was wrong of Stosur to circle it, but to her credit, Errani did not exactly go to pieces the way other players in her shoes often have done. Still, she lost that game and was broken in her next one, too. Stosur won the set with ease.

Errani went one better than Janko's Law in the third set. She didn't just stay with Stosur, she broke her clearly nervous opponent right off the bat. With her feet growing roots in the court, Stosur netted a backhand to go down 0-40, managed to win the next point, but badly shanked a forehand to give Errani a 2-0 lead.

Stosur managed to break back when Errani's resolve weakened in the fifth game. The underdog produced a flurry of errors, managed to stave off three break points, but just couldn't get over the mental hump while Stosur experienced a rebirth. At deuce, she hit a cross-court forehand service-return winner and followed on with a winner after a brief rally to pull even again.

The deadlock lasted for two more games. But the nerves returned to taunt and haunt Stosur when she served at 3-4. She fell behind 15-40, fighting herself all the way. She pulled it together to dispel two break points, the second with a bold backhand winner down the line. But at deuce she whacked a double fault, and in the next break point she made an inside-out forehand error off the service return to drop her serve. In position to win the match, Erani did not hesitate at all. She drove the dagger home with four straight points.

Given that Stosur's serve averaged 20 M.P.H. faster than Errani's through their previous matches, the amazing statistic in this match was Errani's 86 percent first-serve conversion rate. The outstanding number didn't yield as many easy first-serve points as Stosur's lower but far more deadly 71 percent, but Errani's ability to win with her first serve kept the favorite somewhat off balance, and the Italian's greater consistency (Stosur had 48 unforced errors to a mere 21 by Errani) and—above all else—mental strength took care of the rest.

Given the circumstances—semifinal debutante, intimidating opponent, tough conditions—Errani played a magnificent match. Score another one for the mind.

—Pete Bodo