Henin set high standards as a junior competitor, taking the Junior Orange Bowl international tennis championship crown in Miami in 1996 and winning the French Open junior championships the following year. She turned professional on Jan. 1, 1999, at age 16, and in 2000 she finished among the top 50 players in the world. She married Pierre-Yves Hardenne in 2002, and she competed as Justine Henin-Hardenne until her divorce in 2007. During the 2003 season, she captured two major championships, ousting fellow Belgian Kim Clijsters in the finals of both the French Open and the U.S. Open (United States Open Tennis Championships); her victory on the clay courts at Roland Garros in the French Open marked her first grand slam title. Moreover, she was victorious in 75 of 86 matches, winning 8 tournaments altogether and reaching the semifinals or better in 16 of the 20 events she entered. Her remarkable consistency and determination enabled her to finish the year as the top-ranked player in the women's game.

Henin's run continued with a victory at the 2004 Australian Open, but a virus sidelined her in the weeks leading to the French Open. Her departure in the second round at Roland Garros made her the first top-seeded woman at the tournament to lose before the third round. She did not compete again until the Olympic Games in Athens. Remarkably, she rescued herself from 1–5 down in the final set of her semifinal against French Open victor Anastasiya Myskina of Russia and then took the gold medal over Amélie Mauresmo (Mauresmo, Amélie) of France. At the U.S. Open two weeks later, however, Henin was ousted in the fourth round—the first time since 1980 that a number-one seed had been beaten before the semifinals in that tournament. Soon after, the Belgian champion announced that she would not compete for the remainder of the year.

The 2005 French Open saw a return to championship form for Henin, who entered the tournament as the 10th seed. She reestablished her dominance of the clay at Roland Garros with championships in 2005 and 2006, and she reached the finals of every other grand slam event in 2006. Henin did not compete at the 2007 Australian Open, in order to handle her divorce, but several months later she earned her third consecutive French Open victory, becoming only the second woman to accomplish that feat since 1937. She also set a French Open record by winning 35 consecutive sets. In 2007 Henin won her second U.S. Open title. She stunned the tennis world in May 2008 when she announced her immediate retirement from the sport, despite being ranked the number one player at the time. Henin cited fatigue and a recent stretch of poor play as reasons for her retirement, and she left the game just two weeks before she was to defend her title at the French Open.