Before Justine Henin became the world no. 1 in tennis and perhaps the best pound for pound player the women’s game has ever known, before she won three consecutive French Opens and reached the final of all four major tournaments in a single year, she had to make it in New York.

At the U.S. Open four years ago, Henin, the 5-foot-5-inch, 125-pound Belgian dynamo with the most beautiful backhand in tennis, had only one Grand Slam title to her name, and many considered her fragile, not just in body, but in mind. The fans at Arthur Ashe Stadium, hungry for tennis after rain had washed out much of the tournament, were more than willing to test this theory. They filled the stadium for a hastily scheduled Friday evening session and pulled for Jennifer Capriati, America’s own comeback kid, to reach her first U.S. Open final. They booed Henin. They hollered at her. One woman, seated in the stadium’s upper bowl, shouted “double fault!” every time Henin began her second serve.

It wasn’t quite the raucous scene of the 1977 U.S. Open, the last one in Forest Hills, but hospitable it was not.

Henin blocked out the noise, the taunts, and the pain from leg cramps that later left her lying on a trainer’s table with an intravenous tube hanging from her left arm. At 2 a.m. She trailed 5–3 in the second set and rattled off four straight games. In the third set, she again trailed 5–3, and her cramps made serving painful. She thought about calling for a trainer, but her controversial encounter with Serena Williams at the French Open earlier that year, when Henin signaled she was not ready to return a serve and then denied it, flashed through her mind. She played on. Ten times she came within two points of losing the match. Ten times she escaped until she prevailed 4–6, 7–5, 7–6(4) in one of the U.S. Open’s most memorable matches. The next night, she won her first and only U.S. Open title.

A different Justine Henin will arrive in New York this week. She is now 25 years old, divorced, and in contact with her father and siblings, from whom she strayed in the years following the loss of her mother, who died of cancer when Henin was 12. Her sportsmanship remains questionable at times: Just last year, she retired in the Australian Open final and robbed Amélie Mauresmo of a match point keepsake from her first major title, and then retired again in the last doubles match of the Federation Cup team competition, which handed Italy the title and left her Belgian fans in dismay.

Her tennis, however, has never been better. For the last two years, Henin has outplayed every other woman in the world. She reached the finals of all four majors in 2006, and she won the first one she played this year. In 10 tournaments this year, Henin has six titles and a record of 43–4. Last year, she was 60–8 with six titles, including the season-ending championships in Madrid.

Yet despite all her successes, it’s Henin who has the most to prove at this year’s U.S. Open. As well as she has played the last two seasons, she hasn’t shown the toughness she showed in 2003, at least not when she most needed to show it. At Wimbledon last year, she won the first set against Mauresmo and lost the match. This year, she defeated Serena Williams at Wimbledon, but suffered a devastating loss in the semifinals to Marion Bartoli, an unheralded and overweight Frenchwoman. At last year’s U.S. Open, she could not withstand the firepower of Maria Sharapova.

Henin has already won enough Grand Slam titles — six in all — to earn her a place among the all-time greats of the game. But it’s of no small significance that she hasn’t won a major other than the French Open since 2004, when she won her first and only Australian Open. Considering Henin’s size and history of injuries, her time as a Grand Slam contender might be up in two or three years. And if this visit to Flushing isn’t her last chance for another U.S. Open title, it may well be her best one, even if the sore shoulder that nearly caused her to default in Toronto last week flares up.

Other than Henin, there are no consistent threats in women’s tennis these days. The Williams sisters either dominate or disappear (see above); Sharapova has struggled with her right shoulder and pulled out of her last tournament with a leg injury; Mauresmo won’t play the U.S. Open because of lingering ill effects from an appendectomy earlier this year. The old-guard Russian women who received so much attention at the U.S. Open the last few years — Nadia Petrova, Svetlana Kuznetsova, Anastasia Myskina, Elena Dementieva, and Vera Zvonareva — are now a collection of also-rans (the injured Myskina, the first Russian to win a Grand Slam title, is working in television and contemplating retirement).

Amid all this uncertainty, two Serbian women, Jelena Jankovic and Ana Ivanovic, hope to make the leap from contender to champion, just as Henin and fellow Belgian Kim Clijsters, whom Henin beat in the 2003 final, hoped to do four years ago. Jankovic covers the court better than anyone on the tour save Venus Williams. Ivanovic doesn’t move as well, but she’s a natural slugger with polished strokes, sound volleys, and a powerful, if at times unreliable, serve.

Of all the young women on the tour, Ivanovic, who learned much of her tennis on a court built inside a drained swimming pool, has the most potential. She reached her first major final at this year’s French Open and followed it up with a semifinal appearance at Wimbledon. Ivanovic looked strong in winning her first hard court tournament of the summer, but in Toronto last week, she lost to a Chinese qualifier ranked 169 in the world (she won the title in 2006).

So far, Henin has almost singlehandedly prevented a Serbian breakthrough: She has beaten Jankovic and Ivanovic nine times overall and three times in the semifinals or final of a Grand Slam tournament. Perhaps in two weeks she’ll prove to them that she still has what it takes to win in New York.