Nine months into her comeback from a three-year retirement from tennis, Martina Hingis seems to have reached the limit of her abilities.

Hingis, the former world no. 1 known for her stylish play and sharp tongue, has had a respectable campaign in 2006. At the Australia Open, her first major tournament since 2002, she reached the quarterfinals and took a set off the no. 2 seed, Kim Clijsters. She defeated Maria Sharapova in Tokyo, won a title on clay in Rome, and made the quarterfinals of the French Open. After her upset in the third round of Wimbledon, she bounced back with a trip to the finals of the Rogers Cup in Montreal (she lost badly to Ana Ivanovic).

For a newcomer or an aging veteran, this would amount to a fine season. Why complain about a 43–14 record and a tad less than $1 million in prize money? For the 25-year-old Hingis, however, the good results are not good enough — that’s why she left the game in the first place (along with repeated injuries to her feet). Back then, the Williams sisters and Lindsay Davenport were overwhelming Hingis with their increasingly powerful and deep groundstrokes. Though those three Americans now suffer from either injuries or mental burnout, and many other top athletes are hurting, little has changed for Hingis.

Today, she still must live with a simple, unpleasant fact: When the game’s best athletes are healthy, she has no chance. Against players ranked inside the top 15, Hingis is 10–11 on the season. Her record against women ranked inside the top 10 is 4–9. Besides Sharapova, her top 10 wins have come against Davenport, who has been injured most of the season, and Elena Dementieva and Svetlana Kuznetsova, two supremely capable, and supremely inconsistent, Russian ball bashers.

It’s common to attribute Hingis’s disadvantages to her size. There’s truth to this assertion: It’s much worse to be 5-foot-6 (she is generously listed at 5-foot-7) and 130 pounds than, say, 5-foot-9 and 152 pounds, like Amelie Mauresmo, the current no. 1 player in the world.

Still, size is not the entire problem. It’s also a function of conditioning — endurance training, as well as lifting weights — and, to some extent, technique. There isn’t a shot that Hingis can’t hit, and her form is impeccable, almost Chris Evert-like, whether it’s a forehand, a volley, or a compact two-handed backhand. Yet her shots rarely have the explosive burst, akin to “life” on a major league fastball, as those of her colleagues. While she should not be expected to thump the ball like the 6-foot, 160-pound Ivanovic, she could be getting more out of her body.

The most apt comparison is Justine Henin-Hardenne, the no. 2 seed and 2003 champion, who won her first-round match yesterday over Maria Elena Camerin 6–2, 6–1. The Belgian is a year younger than Hingis and not much taller than 5-foot-5 (the tour’s media guide gives her a three-quarter inch bonus). She, too, weighs 130 pounds.

Yet Henin-Hardenne’s strokes, especially her forehand, jump off her racket. Her serve, the shot for which height plays its most important role, has become one of the strongest in the game, both in terms of speed and spin.

Why? Genetics could have much to do with it. Henin-Hardenne also has trained harder than Hingis in terms of weights and conditioning, and she was working out daily through her most important years, her early 20s, when Hingis was off the circuit. Henin-Hardenne spent years working with Pat Etcheberry, a man renowned for his punishing routines, and is as strong as anyone on the tour (pound-for-pound, that is).

As for technique, John Yandell, a long-time tennis instructor and the founder of Tennisplayer.net, which uses high-speed video to analyze players and their strokes, noted some subtle differences between Hingis and her doppelganger.

Henin-Hardenne is in the minority of women who serves with a platform stance: both feet remain in place before launching up and into the ball (in a pinpoint stance, a right-handed player either drags or quick-steps her right foot alongside her left before liftoff). This technique is far more common among male players and the game’s best all-time servers, including Pete Sampras. Yandell believes that a pinpoint stance disrupts what he calls the “biomechanical chain,” essentially limiting the use of one’s legs and reducing torque (the twisting of one’s torso) during a serve. This reduces racket speed.

When she first came onto the tour, Henin-Hardenne used a pinpoint stance. About four years ago, her coach, Carlos Rodriguez, asked Yandell for footage of Sampras’s serve. Henin-Hardenne has since remodeled her serve and turned it into one of the most effective on the tour. She still has bouts with double-faults and, occasionally, wild ball tosses, but her potent first strike has helped her prevent much bigger and stronger players from attacking her as soon as a point begins.

Rodriguez requested some footage of Andre Agassi’s forehand, too. Yandell said that Henin-Hardenne tends to stretch her left arm farther across her body when she prepares for a forehand, coiling up more and eventually unleashing more energy during her swing. She also tends to extend through her swing more than Hingis.

When Hingis retired, she had a pinpoint stance on her serve. She has changed to a platform stance since returning in January, a move that Yandell applauds. Her serve remains a weakness, however. While genetics and lost time have played their part, Yandell said a difference in mental attitude, forged over years (in Henin-Hardenne’s case) of slinging rocks at the tour’s Goliaths, should not be underestimated.

“Justine just appears more capable of aggressive shot-making,” he said. “I think it’s difficult to equate differences in performance directly to technique. Great players with technical flaws will usually defeat less talented players who may have better stroke patterns. You can look at Venus Williams’s serve or forehand for an example of that.”

This afternoon, Hingis returns to the U.S. Open for the first time since 2002, and she’ll have a few chances to prove that she has put the past behind her. Her first opponent is no walkover: Peng Shuai, a hard-hitting 20-year-old from China. From there, she could face Dinara Safina and either Ivanovic, Mauresmo, or Serena Williams.

It’s not a draw that Hingis of old would have overcome, but maybe this year will be different.