The 23-year-old Belgian, once ranked No. 1 and winner of the 2005 U.S. Open, said on her Web site it was time to stop immediately after playing for 10 years and start a "new life." She won the 2002 and 2003 WTA Championships as part of her career 34 singles titles.

Clijsters is now ranked No. 4. She lost in straight sets to Ukrainian qualifier Julia Vakulenko on Thursday in her final match, failing to retain her title in Warsaw, Poland.

"It has been more than fun, but the rackets are being hung up," she said. "To retire before the age of 24, it is very young _ but it was so beautiful. I would have been able to continue for a few months and to take part in the four most lucrative tournaments (three Grand Slams and the Masters).

"Money is important, but not the most important thing in my life. Health and private happiness are so much more important."

She said her struggles with injury had taken its toll and she wanted to focus on other things, including her July 14 wedding to Brian Lynch, a former Villanova basketball player now playing in Belgium.

"It's tiring to get out of bed and to use an hour just to warm up stiff muscles in the morning," she said. "The constant injuries and continual rehabilitation ... it makes it all even more difficult to go on."

A left wrist injury last year stopped Clijsters from defending her only Gland Slam title at Flushing Meadow and from playing in the Fed Cup final. She reached the 2007 Australian Open semifinals but injured her hip. She recently complained of back pain after losing in Key Biscayne, Fla.

Clijsters was Belgium's first tennis player to reach No. 1, holding the top-ranked spot for 19 weeks in 2003. But she was unlucky in Grand Slam finals _ losing in 2001 and 2003 at the French Open, in 2003 at the U.S. Open, and in 2004 at the Australian Open.

"The most beautiful memories of my career?" she wrote on her Web site. "Many victories in tournaments, Grand Sams in singles and doubles as well as being world No. 1 in singles and doubles.