Top-seeded Serena Williams underwent an interesting transformation in her quarterfinal with Ana Ivanovic at Stanford tonight. Starting out, she was the unsure, out-of-sync, almost morose champion we’ve seen all too often this year. In the ensuing hour and 57 minutes, though, she transformed herself into the combative, fearless, alpha female who’s won 17 Grand Slam singles titles and doesn’t appear to be sated yet.

Williams certainly hit a few rough patches along the way, but you can give the fifth-seeded Ivanovic a lot if not all of the credit for that. In the end Williams throttled her quick and extremely dangerous rival — a Grand Slam singles champ herself and former No. 1 — in a real corker, winning for the sixth time in seven often close matches, 2-6, 6-3, 7-5.

This was just the second match Williams has played since she was stunned by Alize Cornet in the third round of Wimbledon way back in late June. There were kinks to get out, and plenty of them.

These days, it seems the woman almost feel obliged to roll out their big guns at the very first opportunity, and to keep them firing until the joyful — or bitter — end. Williams began today’s match with aces on two of her first three serves. Three points later, she was down a break. That’s how it is today; it’s as much an arms race as a tennis match. Ivanovic has been hitting the ball with crisp, fierce precision lately, and today was no exception. She quickly consolidated the break for 2-0.

Williams was undeterred; she kept battering away even though she had to stare down a break point in her next service game. She held, though, as did Ivanovic for 3-1. The 26-year old Serbian picked up right where she left off when facing the Williams serve — pouncing on every opportunity, smacking back precise returns from right atop the baseline to claim the first-strike. It was a formula that had worked beautifully for Ivanovic in her fourth-round win over Williams at the Australian Open in January.

The bold strategy paid off again in the fifth game. Ivanovic opened the assault on the Williams serve with an unreturnable forehand service return, and then reached break point with a similar, scorching return — this time from the backhand. Her work was done, because Williams donated the break with a double fault — a testament to the jitters she must have been feeling.

Ivanovic held for 5-1, and had yet to lose a point when she was obliged to hit a second serve. It spelled trouble for Williams, who then played her fourth consecutive service game in which she showed Ivanovic a break point — this one a set point at 30-40. An unreturnable first serve took care of that, and Williams went on to hold. But she was clearly growing tetchy, as evidenced by the two costly, wild errors — a backhand and forehand — that she hit from 30-all. Those prodigious errors gave Ivanovic the first set.

Savvy tennis fans know that one of the first things to look for after the end of a set is how well the player who won it keeps command. As is often the case, in this one Ivanovic didn't assert her control. Williams held to start the second set, and then broke to build  2-0 lead. She was coaxing along her own game, and the breathing room she created made the job easier. She didn’t play a service game in the first set in which she didn’t offer up at least one break point. In the second set, she didn’t give Ivanovic a single break opportunity.

To further complicate matters, Ivanovic appeared to pull a hip muscle and asked for the trainer while trailing, 2-4. As soon as Williams had that game, Ivanovic was evaluated, stretched, and sent back onto the battlefield. She bravely held for 3-5, but Williams served out the set convincingly when, from 30-all, Ivanovic made two unforced errors.

Up to this point, and slightly beyond it, the match had proceeded in an orderly manner. What surprises it had in store were all back-loaded into the third set. By then, the sands of time seemed to be running out on Ivanovic, as Williams gradually adjusted to the pace of the court and that of Ivanovic’s shots. When she broke in the first game of the third set, Williams appeared to have established control of the match. It was soon 3-1 to Williams, and in the next game she was one swing away from going up by two breaks.

But Williams missed a nicely set-up but poorly executed finisher — a cross-court forehand volley. Ivanovic went on to hold for 2-3.

Then, in a dazzling few moments, the tables abruptly turned. Ivanovic broke Williams, then held to lead 4-3. Williams regained her composure and held, then broke Ivanovic to find herself serving for the match. She played a horrible game at that juncture, smacking her seventh double fault to go down 0-30, a deficit from which she would never recover. Soon it was 5-all.

But it was Ivanovic’s turn to stumble. She made three errors to fall behind love-40 and won just one more point in the next game. That left Williams serving for it — again.

This time, her hand was firm and her will granite. Ivanovic hit a poor forehand approach to start things off, and two aces later (No. 9 and 10) Williams had triple match point. She converted the second one, delivering an unreturnable serve to Ivanovic’s forehand. Serena will play No. 8 seed Andrea Petkovic for a spot in the final.