“It’s up to Serena.” Through good times and bad — from no. 1 in the world to a badly out-of-shape underdog at the Australian Open two years ago — this has been the common refrain about Serena Williams. If she’s motivated and wants to win, the thinking goes, she’ll win. At her best, she’s so good that her opponents have no say, and barely any reason to even show up for a match.

Tempting as it has been to see Williams in this way over the years, it hasn’t been an honest assessment for a long time — at least for as long as Justine Henin, the world no. 1 who unexpectedly retired this month, was at her best. No player gave Williams as much trouble as Henin, not even her sister Venus, whom Serena has beaten in seven out of their last nine meetings. Henin had the perfect antidote to Serena’s strength and power: Touch, variety, speed, and supreme concentration. On clay, Henin’s superior footwork gave her a distinct advantage, but she proved last year that she could be better than Williams on any surface when she defeated Williams in all three majors, none more impressive than her 7-6(3), 6-1 victory at last year’s U.S. Open.

At the French Open, which begins Sunday, we’ll see Williams in something close to total control for the first time since her most dominating days on the tour. When Henin said goodbye to the game and forfeited a chance to win a fourth consecutive French Open title, she put Williams at the forefront of women’s tennis again. Though she will only be the fifth seed when the French Open begins, Williams is as much of a favorite as one could pick in a field in which the second-, third-, and fourth-ranked players have won a total of one major title among them.

Much else falls in Williams’s favor this year. She’s slimmer. She’s playing with more enthusiasm than she did in the years following her dominant stretch in 2002 and 2003. She’s also played more clay matches than she normally does by this point in the season. Last year, Williams played three matches on clay leading up to the French Open, where she lost in the quarterfinals to Henin. The previous two years, she missed the French Open altogether. This year, though, she’s played 10 matches on clay, despite retiring from a warm-up tournament in Rome with a tight back (an injury that should have no lingering effects).

With Henin gone, the biggest challenge for Williams might be suppressing her own nerves. If one were to highlight the differences between the 21-year-old Williams — who won five out of six major titles — and the 26-year-old Williams of today, one important distinction would be her hesitancy during important points. The Williams of old kept pummeling forehands and backhands when she neared the finish line. Today, she sometimes loses control, as she did against Jelena Jankovic in the final of the Sony Ericsson Open last month. Williams prevailed, but not without a struggle. On clay, where her opponents will have slightly more time to retrieve shots and annoy her, she’ll need to play with more confidence when under pressure.

Jankovic has the best chance of any of the other women in the field. Last weekend, she defended her title in Rome and she seems to be over the niggling injuries that hampered her in the early part of the season. Jankovic has a great game for clay: a powerful two-handed backhand, a topspin forehand that’s sometimes too loopy on hard courts, but bounces high on clay, and a weak serve that isn’t as easily punished in Paris. If not for Henin, Jankovic might have won the French Open last year in an all-Serbian final (she lost in the semifinals to the Belgian, who dumped the nervous Ana Ivanovic two days later). Jankovic has played Williams to a standstill (they’ve each beaten each other three times apiece), and is particularly adept at blunting Williams’s power and returning her serve, as she showed in the quarterfinals of the Australian Open earlier this year.

But does Jankovic — or for that matter, Ivanovic — have the nerve and confidence to win a major title? This is the biggest question about the two Serbs, both of whom have lifted their country from deep in the basement of tennis to the game’s top floor. Ivanovic has more experience, having reached the French Open final last year and the Australian Open final this year. Though she didn’t win a set in either match, she did play more comfortably in her second appearance on a big stage. Ivanovic has a more well-rounded game than Jankovic and her long strokes work well on clay, where she has time to take a full backswing. She doesn’t have the speed of Jankovic, but her quickness and fitness have improved since this time last year. Her confidence can’t be high, though, not after a loss in the Berlin semifinals (she won the event last year), and another in the second round in Rome to qualifier Tsvetana Pironkova.

If either Ivanovic or Jankovic win the French Open, it would be the first Grand Slam title for a woman from Serbia. If Maria Sharapova (the top seed) wins, it would complete her collection of major tournaments, which she added to earlier this year in Australia. The 21-year-old Sharapova ought to win this title one day, but I’d be surprised to see her do it this year, if only because she doesn’t seem convinced that she’s ready. After winning in Melbourne, Sharapova talked about winning at Roland Garros in the future, when she’s stronger and more fully developed. She’s not going to feel like the top seed, and no one else in the draw will look at her as such, either. Williams certainly won’t, not when she has a chance to take control of the sport in a way that she hasn’t for six years.

Mr. Perrotta is a senior editor at Tennis magazine. He can be reached at tperrotta@tennismagazine.com.