UPDATE 2-Tennis-World number one Henin retires at 25

(adds Federer reaction)

By Darren Ennis

BRUSSELS, May 14 (Reuters) - World number one Justine Henin announced she was retiring from professional tennis with immediate effect on Wednesday.

"It's the end of a wonderful adventure but it's something I have been thinking about for a long time," the Belgian told a news conference.

Henin, the winner of seven grand slam titles, had been expected to defend her French Open crown later this month but instead opted to turn her back on the sport at the age of just 25.

"I am leaving as the world number one and that is important and it is always better to go out at the top," said Henin, who won 41 titles since turning professional in 1999.

She also became the first woman to quit the sport while ranked number one.

"I have been driving my career based on an emotion but I didn't feel that emotion anymore since (last year's season-ending championships in) Madrid," said Henin, whose final match turned out to be a third-round defeat by Dinara Safina in Berlin last week.

"At Madrid I felt I had reached the climax of my career. I had thought about taking a break, but in the end I didn't think this was the right decision.