Tennis is a game that demands brilliance. All of the sport’s greats have been virtuosos of one kind or another. All had supreme powers of concentration and, at their finest moments, unshakable confidence and conviction. Without these attributes, a player cannot hope to conquer one exceptional athlete after another in one-on-one competitions that span 10 months out of every year.

Unfortunately, as tennis has often taught us, brilliance is difficult to sustain. One moment you are Bjorn Borg, four-time defending champion at the French Open and three-time defending champion at Wimbledon; the next, you are finished, your spirit broken by an up-and-comer — in this case, John McEnroe. It happened to McEnroe, too: In 1984, at age 25, he won two Grand Slam tournaments and lost a mere three matches. Though he played another seven years, he never won another major.

Rather than play on with little desire, and with little hope of scaling the heights she once scaled, Justine Henin, 25, yesterday chose to follow in the path of Borg and call it quits. Never before had the no. 1 player on the women’s tour retired. That she did so a few weeks before the French Open — where she had won three years running — and before Wimbledon, the only major title she had never won, showed the state of Henin’s mind.

Henin had lost several lopsided matches this season, but it was her most recent defeat, to Dinara Safina in Berlin, that inspired concerns about her condition. She seemed singularly lacking in confidence and claimed she was fatigued, despite playing little tennis this year. As it turns out, she was tired of trying to convince herself to continue a career that, to her mind, can bring her no titles or honors more important than the ones she has already won. She said she had debated retirement since the end of last season, a year that included her two finest moments as a professional: A French Open title in front of her formerly estranged family, and a U.S. Open title that included consecutive — and convincing — victories over Serena and Venus Williams, the two most talented and iconic players of Henin’s generation (and for that matter, many other generations). Now, she said, she was relieved to put tennis behind her.

“I decided to stop fooling myself and accept it,” Henin said at a press conference in Belgium, where she sat alongside Carlos Rodriguez, the coach and de facto psychologist who coaxed more incredible tennis out of Henin than anyone else could have done. Rodriguez, like Henin, shed a few tears as he spoke.

“Because of her,” he said, “I am somebody.”

The news of Henin’s retirement was a shock to everyone in tennis, but only because it came so abruptly and with her favorite tournament around the corner. After a few minutes to digest the news, it began to make sense. I witnessed this quick transition from shock to comprehension when I spoke to former world no. 1 Tracy Austin yesterday in conjunction with another story. It was early California time, and Austin had not heard the news.

“Retired!” Austin said. “Are you kidding me? Holy smokes! Retired from the French Open, or retired from tennis altogether?”

It wasn’t more than a few minutes, though, before Austin had gathered her thoughts. “She dug so deep in matches, but physically she was at such a disadvantage to everyone else,” Austin said. “When she says the word ‘relieved,’ that says to me that she’s done. That means you don’t have any desire to fight anymore.”

Henin played beautiful tennis — her one-handed backhand was the most stylish shot in the sport and her footwork second to only one, that being Roger Federer — but unlike most tennis that was beautiful, hers did not seem effortless. Henin worked tirelessly to produce flawless shots. She suffered numerous injuries (inevitable when a 5-foot-5-inch, 126-pound women tries to trade blows with 6-foot, 160-pound giants). More than anything, she struggled with her confidence. Before Henin won her first major title, many saw her as a “choker,” a talented player who couldn’t deliver under duress. Instead, through will and with enormous help from Rodriguez, she overcame her natural tendencies and won seven major titles. It would be an astonishing accomplishment for any player, never mind one this fragile.

Henin had some terrible moments along the way. Fans of Serena Williams won’t soon forget the time Henin raised her hand at the 2003 French Open, signaling she wasn’t ready to receive serve, and then didn’t fess up to it when the chair umpire failed to notice as Williams casually missed a serve (an angry Williams accused Henin of “lying and fabricating”). Instead of finishing the 2006 Australian Open final against Amelie Mauresmo, Henin, sick to her stomach, quit when trailing 6-1, 2-0.

Yet there was far, far more good than bad. At times, Henin was majestic. Three moments that stand out are her 2003 U.S. Open title, which included a three-set, after-midnight victory over Jennifer Capriati that left Henin dehydrated and in need of medical attention; her 2004 Olympic gold medal, which she won after playing just two matches in the prior four months because of a strength-sapping virus, and last year’s U.S. Open, when she played — by far — the best tennis of her life, dismantling the Williams sisters in the quarterfinals and semifinals.

Looking back, it’s easy to see Henin asking herself after that title, “What else can I do now? Can I play better than that?” It’s not terribly surprising that she couldn’t come up with a compelling reason to work as hard as she had from the day she first met Rodriguez 12 years ago. Sometimes in tennis, the highs are so high that there’s no motivation to sail along at slightly lower altitudes. Considering how much it took for Henin to get to the top in the first place, I’d be surprised if she ever tries to get there again.

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