Justine Henin, the world No 1, has announced her retirement from tennis at the age of 25 with immediate effect.

"It's a big day in my life," Henin said. "I know that it's a shock and a surprise for a lot of people but it's a decision I have been thinking about for some time.

The Belgian headed the official WTA rankings for the 117th consecutive week, but pulled out of the Rome Masters this week blaming tiredness. "It's the end of a great adventure, the end of something I had dreamed of since I was five," she said, close to tears, alongside her long-time coach Carlos Rodriguez.

Henin, who has seven grand slam titles to her name and almost £10 million in career earnings since she joined the WTA Tour in 1999, has been struggling this year to reach the level of form that saw her claim her last three titles at Roland Garros. She admitted last week she was lacking confidence just two weeks prior to her French Open defence.

"I'm at the end of the road," Henin, the winner of 41 singles titles, said. "I have experienced everything I could have. I have lived completely for tennis. I am relieved and proud of what I achieved."

She insists her decision is final and she will not make a U-turn at any point in the future. "A new future is ahead and I won't go back on this decision."

Henin has been beaten four times in four months in 2008 including a 6-2, 6-0 thrashing by Serena Williams in Miami last month. Her latest defeat and last appearance on court came at last week's German Open where she made a shock exit when coming off second best in a two and a half hour third round marathon match against Dinara Safina, of Russia.

Larry Scott, the WTA Tour chief executive, paid tribute to the Belgian. "Justine Henin will be remembered as one of the all-time great champions in women's tennis, and a woman who made up for her lack of size with a will to win and fighting spirit that was second to none," he said. "It is rare that an athlete leaves at the very top of her game in this day and age, but Justine has always played by her own rules, in the very best sense of those words."

"History will remember Justine for not only her seven grand slam titles and three years finishing as the Sony Ericsson WTA Tour's world No 1, but for one of the most graceful backhands the sport has ever seen and an ability to overcome any and all obstacles placed in her way on the tennis court and off.

"This is a sad day for our sport and for her millions of fans around the world, but I know that I speak for so many in wishing Justine the very best in her future endeavours and hope that she will stay connected to the sport to which she gave so much."

Henin has previously admitted she would like to have more time to study, travel and generally escape from the pressures of top-level tennis, but her retirement was a surprise to Sabine Appelmans, the Belgium Fed Cup captain.

"This is for me a shock," Appelmans said. "I find it a pity but we must respect the choice. Everyone in their tennis career goes through periods with motivation problems. I understand well after her heavy season last year but I would take a period of rest and look from a distance at what is happening.

"Quitting is very drastic. Henin is a big champion and they must part in style. It's a shame that she must depart now. It's a pity that she can't cope with the defeats."