MIAMI, Fla.—I don’t know how you say “anarchy” in Serbian, but it wouldn’t surprise me if it were spelled J-A-N-K-O-V-I-C. Is there a player out there who can so blithely and comprehensively upset the beautiful internal logic and orderliness built into this game? Jelena Jankovic ought to be waving a wand instead of a tennis racquet, and muttering incantations over a steaming cauldron instead of arguing with the chair umpire. Her matches ought to be played on a Ouija board.

It sometimes seems that no situation is so desperate that Jankovic can’t pull out a brilliant inside-out backhand, or full-rotation forehand, to make you think, “yes”—just when you were just about to decide, “no.” No lead is so big that she can’t blow it. No commonplace moment so insignificant that she can’t transform it into a turning point. Take the beginning of the second set in her quarterfinal match in the Sony Open with WTA No. 15 Roberta Vinci on Wednesday night.

The backstory: Jankovic fell behind in the first set, 0-2. She clawed and scratched her way back and broke for a 4-3 lead, only to be broken back, thanks mainly to a pair of double faults. She managed to break again, though, and served out the set with admirable—and uncharacteristic—calm and command.

It took Jankovic just moments to pin Vinci down, 0-40, in the first game of the second set. But she let the server slip the noose and save the game. In the next game, Jankovic served her way to a 40-0 lead—but she lost the next four points in succession, and ultimately the game as well.

This was pure anarchy, or Jankovic. It’s the kind of stuff that tempts you to throw your hands in the air and walk away from the whole stinking thing—if only that butter-smooth athleticism and wonderful racquet work wasn’t so bewitching. If she hadn’t always been a player of such exquisite promise alongside such baffling, self-sabotaging tendencies. . .

But tonight ended well once again for Jankovic, the way it used to with far greater frequency. She made a hash of the second set and fell behind in the third 0-2, but eventually turned it around with a break at 4-2 and held on to serve it out, 6-4, 6-7 (6), 6-3.

Jankovic finished No. 1 for the year in 2008, but without having won a Grand Slam title. She chose to train in Mexico in that off-season, and it proved a disaster. She worked herself into such good shape that she ended up feeling muscle-bound and a half-step slower. She started poorly in 2009 and never really recovered her equilibrium. Her ranking slipped down as low as No. 34 (August 2012), but she’s vowed to turn that around.

“I’m feeling better and playing better,” Jankovic told me after wresting the win from Vinci. “I feel in good shape and it’s great to get this feeling back, and to be in this situation (playing Maria Sharapova in the semifinals) again. I want to be up there again. I’m striving every day. Hopefully my time is coming—I feel it coming and hopefully I can get there.”

Jankovic appeared to take a significant step in the right direction, outdueling a woman presently ranked seven rungs above her. Vinci won their last two meetings, and her slice-heavy style troubles Jankovic.“It’s always tough because her game doesn’t suit mine. It’s difficult for me to get under her ball and just go after those slices aggressively. It wasn’t an easy match, physically or mentally.”

One thing the match-up did prove, at least to my satisfaction, is that any notion that a woman can rely primarily on a slice backhand and survive among the elites of today’s game is a romantic fiction. As much trouble as Jankovic had at times with Vinci’s slice—and the Italian’s cutter is about as good as the shot gets—the reply was always in the Serb’s hands. The shot, whether forehand or backhand, was always there to be hit.

I also noticed in reviewing her history that for all her trepidations, Jankovic actually won the first three matches she played against Vinci (all between 2004 and late 2009). That suggests that the real problem these last few times might have been nerves; even a well-hit slice is tantalizing, and a challenge to a returner’s confidence—which is precisely why a focused, confident player today has nothing to fear from it.

Jankovic isn’t quite there yet. She may never get there, at least not to the same degree as some others. For better or worse, she has that drama gene. At this point, it’s unlikely that she’s going to outgrow it and become a steady, stable, predictable performer who will find her rankings niche and camp there. She’s still capable of great things, I think. But inspiration and desperation seem destined to be intertwined in her history.

It was like that tonight. When I asked why it always seemed to be such an adventure out there for her, she replied: “Well once again it was me getting a bit emotional out there. At times feel it’s all in my control, up to me if I’m going to play well. So I do. But then I get too erratic. I make too many errors. I do things I shouldn’t. I get frustrated with myself. Then all I can do is re-group. I think I was able to do it this time. I managed to stay stable and finish off the match.”

Tomorrow, of course, is another, new day. Predict her fate if you dare. I’m going to make mine with the help of a Tarot deck.