IN RUSSIA, says Garry Kasparov, your 40th birthday is not considered a matter for celebration. This seems perfectly reasonable to me, but Kasparov, the youngest world chess champion, who held the title for 15 years, nevertheless chose to celebrate his.

The morning after at Home House, his fashionable, marbly London club, he, like his agent and the Russian Sports Minister, is a little blurry around the edges. If you fancied a shot at beating history’s grandest grandmaster, you would probably never get a better chance.

Whatever the excesses induced by the wine and the 23 close friends who flew from 11 nations to toast him, Kasparov still looks like a champion today — although he no longer is one. Muscles honed in the gym gather beneath his jumper. Tufts of wrist-hair emerge from its cuffs. His dark, intense eyes are trimmed with red, but they immediately seduce The Times’s female photographer.

He has been called The Beast of Baku, after his birthplace, the capital of Azerbaijan, but he is a seductive interviewee too, his voice resonating with a thunderous exuberance. He clearly considers himself an international figure whose intellect would be wasted if confined to the borders of Russia, let alone a chessboard. If I didn’t know, I’d guess he was the master of something, but never the costive, sedentary, taciturn game of chess.

“But chess,” he corrects me, “is theatre. It is a form of fatal attraction. It is psychological warfare: two characters, two clashing intellects.”

The Kasparov career breaks into unequal halves: rise and falter.

Acknowledged as a chess genius at the age of seven, he won the world junior championship at 17. When, in 1985, he wrested the world title from the Soviet grandmaster Anatoly Karpov, it was seen as a victory for youth and reform against the USSR’s obdurate sports committee and its sponsors, the Communist Party and the KGB, both of whom he battled unceasingly. He beat Karpov three more times, surpassed Bobby Fischer’s record as a grandmaster, and in 1993 saw off Britain’s Nigel Short, who thought him “a very unpleasant man” who needed to be beaten, and the sooner the better.

Then, in 1997, he was defeated — by a computer. Deep Blue annihilated him in 19 moves, removing his aura of invincibility and chipping his confidence. Three years later, at the Riverside Studios in London, he lost his world title to Vladimir Kramnik, the ultimate bespectacled chess nerd.

Kasparov says that the problem was that he had won too many tournaments, and a player can only learn from defeat. It took him two months to recover his spirits, analyse his errors and go on to win so many games that he kept his top ranking. But we knew that he was fallible now.

Last year a woman, one of that breed that he had claimed could never play great chess, beat him. Then, this February, he shocked his fans by settling for a draw against the latest super computer, Deep Junior. His style, he had once said, was always to grab “the chance for greatness and the chance for disaster”. Now he followed the humble agenda of not losing. The critics said that he had been spooked.

“In the world of chess,” he explains to me, “they have always looked at any failure in my career and said, ‘That’s it’, and written me off. My mother always tells me, ‘Garry, you have too many enemies in the world of chess, they keep you going’. I have been around so long. Many people don’t believe I am 40. They hear my name and think I must be at least 50.”

How does it feel for the child prodigy to be 40?

“That’s an interesting question. When I won my title in 1985 I was 22. I made some remarks that maybe at age 30 I would do other things: you know, study history, study philosophy. For me 30 was already old. When I reached 30 I thought it was an eternity before I turned 40. Now I feel like it’s not about age; it’s about your feelings. It’s whether you are still determined, whether you still have the passion for your game and your energy.” His determination and passion remain constant, he says, and if his energy is less than it was, then he wastes less of it because he knows more. This dividend of ageing is sometimes called wisdom. In chess terms, it is sensing the future gains to be made from the temporary sacrifice.