Clijsters was an accomplished junior player. In 1998, she was the runner-up in the Wimbledon junior singles event. She also won the French Open junior doubles event with Jelena Dokic and the US Open junior doubles event with Eva Dyrberg, and finished the year ranked number 11 in singles and number 4 in doubles in the ITF junior world rankings.
In 1999, Clijsters made the breakthrough into the senior ranks of women's tennis. At Wimbledon, she played through the qualifying rounds to make the main draw and beat Amanda Coetzer en route to the fourth round, where she lost to her childhood idol Steffi Graf. Later that summer, Clijsters reached the third round of the US Open, where at one stage she served for the match against, but ultimately lost to, the eventual champion Serena Williams. In the autumn of 1999, Clijsters won her first WTA singles title at Luxembourg, and then her first WTA doubles title at Bratislava, partnering with Laurence Courtois.
She climbed her way up the rankings over the next couple of years. She reached her first Grand Slam final at the 2001 French Open, where she lost an extremely close match to Jennifer Capriati by a score of 12-10 in the final set. Her next important breakthrough came at the end of 2002, when she won the year-end WTA Tour Championships in Los Angeles, scoring a huge win in the final over the world number one at the time, Serena Williams.
Clijsters had one of her most successful years of her career in 2003. She won nine tournaments that year, including the WTA championships, reached two Grand Slam finals at the French Open and the US Open, losing on both occasions to her compatriot Justine Henin-Hardenne, and was ranked number one in the world for several weeks, although she eventually finished the season at number two behind Henin-Hardenne.
Clijsters started 2004 by reaching her fourth Grand Slam final at the Australian Open, where she lost again to Henin-Hardenne; and then went on a very successful run where she won two titles at the Paris and Antwerp. Unfortunately, Clijsters then began to have injury problems with her wrist, which eventually required surgery and forced her to withdraw from the rest of the 2004 Grand Slam tournaments.
In February 2005, after almost a year of inactivity caused by injuries, she made her return to the WTA tour by participating in her home country tournament at Antwerp. She then completed a stunning comeback to the top echelon of tennis when she won, as an unseeded player, 14 straight matches against world's top players to claim two Tier I titles (Indian Wells and Miami) in March, 2005 (defeated five of the world's top six players at that time). In 2005 she won 9 singles events, her last one was at the Gaz de France Stars in Hasselt.
Clijsters has won 30 singles titles in her career so far. Two of those came at the prestigious year-ending WTA Tour Championships, affirming the fact that she is quite capable of winning a tournament featuring only the top eight women players in the world, and another one came at the 2005 U.S. Open.
Clijsters finally got the Grand Slam monkey off her back when she won the 2005 US Open. Clijsters defeated Mary Pierce 6-3, 6-1 in the finals, winning 2.2 million USD — the largest payday in women's sports history. Her actual winnings from the U.S. Open were 1.1 million USD, but because of her #1 ranking in the USTA in 2005, she received a 100% bonus from the association. Along the way, Clijsters defeated both Venus Williams and Maria Sharapova. Clijsters is considered by her peers as one of the most likeable players on the WTA Tour. Her Grand Slam title dispels past criticisms that being "too nice" has prevented her from ever winning a major.
On September 15, 2005, it was announced that the cooperation between Clijsters and her coach Marc Dehous would come to an end. De Hous had brought her during the three years they worked together, to the number one spot on the WTA ranking, two Masters titles and a Grand Slam victory but thought it to be time to do something else. Clijsters will take no new coach but will continue on her own.
Clijsters has reportedly said that she will play for another two years at most because of the toll that tennis has taken on her body.
At the 2005 WTA Tour Championships Clijsters was defeated after only 2 matches. Her first match she lost in a thrilling three-setter against Mary Pierce (6-1, 4-6, 7-6). Amelie Mauresmo defeated her in her second match (6-3, 7-6). Clijsters said in interviews that her defeats are due to tiredness and maybe a jet lag because she only left Belgium friday morning (local time). She won her third match in round-robin against Dementieva with 6-2 and 6-3, but that match was only for the honor because both players were already knocked out the tournament.
Clijsters became the first tennis player (man or woman) ever to rise from outside the Top 100 (No. 134) to No. 2 in the same year.