"I was terrible," Martha Argerich said, after drawing hysterical applause with a recent pair of performances at Carnegie Hall. "I was in a strange state. My mind was on other things." Argerich was sitting in a midtown hotel restaurant, amid a motley, lively group of friends. She had just played Beethoven's First Piano Concerto with the Montreal Symphony. The friends, speaking in various accents of Argentina, France, Hungary, and California, protested that she had in fact been stupendous, colossal, magical. The conductor Charles Dutoit, who is nearly as famous for being one of Argerich's former husbands as for leading the Montreal, joined in the campaign of reassurance as he hastened to catch a plane to Paris. "You were not terrible at all, pas du tout," he told her. But Argerich had made up her mind. "I played better in April," she said. She waved the praise away as if it were a silly dessert.

Argerich, a sixty-year-old native of Argentina, reigns supreme over the feudalistic world of virtuoso pianists. Rivals become mere fans around her, lingering at the door of her dressing room and then skulking away. Her concerts conjure up scenes from another place and time: grown men running down the aisles clutching bouquets, world-renowned musicians pummelling the railings of the upper boxes, jaded critics breaking into foolish smiles. Argerich brings to bear qualities that are seldom contained in one person: she is a pianist of brainteasing technical agility; she is a charismatic woman with an enigmatic reputation; she is an unaffected interpreter whose native language is music. This last may be the quality that sets her apart. A lot of pianists play huge double octaves; a lot of pianists photograph well. But few have the unerring naturalness of phrasing that allows them to embody the music rather than interpret it.

Sensing that my well of superlatives was running dry, I gave up critical objectivity and accepted an invitation to meet the woman in person. With the help of Argerich's daughter, Annie Dutoit, whom the pianist visits when she is in New York, I broke through a crowd of autograph seekers to the inner sanctum of her dressing room. In some cases, the dressing rooms of musical celebrities are places of suffocating sycophancy, but Argerich's had a kind of nervous camaraderie. The usual glad-handers were absent. Argerich was sitting on a couch, her eyes flitting alertly around the room. Her first question was whether anyone had matches. After undergoing treatment for melanoma, which had spread to her lungs, she stopped smoking, but she is delighted when people smoke in her presence.

She is a compact woman, with surprisingly small hands and an enormous mane of long black hair. Her beauty dwells in her large eyes, which are at once sad and sly, and in her smile, which has been compared more than once to the Mona Lisa's. Musicians have a history of falling in love with her and coming away crushed by the force of her personality. It is said that one of her amours would labor for hours over a difficult score, only to watch Argerich, a person of nocturnal habits, slouch downstairs in the middle of the afternoon, rub her eyes, and sight-read the music effortlessly.

Argerich made her début at the age of eight, in Buenos Aires, and she was winning European competitions by the age of sixteen. At first, she pursued the usual international career, but as she matured she became a law unto herself, defying such would-be Svengalis as Arturo Benedetti Michelangeli. Even in the few hours I was with her, I caught a glimpse of her childlike temper; it flared when she felt herself being pushed prematurely toward a decision for which she was not prepared. "I knew this was going to happen," she said, her husky voice rising in volume and descending in pitch. "It is becoming confusing now, it is becoming a big mess." Solutions were proposed; Argerich selected one; the calm of conversation resumed.

Argerich is notoriously difficult to pin down. She cancels concerts, even entire tours, at the last minute, changes programs at will, and generally drives the programming people crazy. She has become a substantial presence in New York in recent years, but only because her stardom has given her unprecedented latitude to schedule events on short notice. This season, she is alleged to be presenting a "Perspectives" series at Carnegie Hall, with programs planned for January, April, and May, although no one is going to guarantee anything until she is actually seen sitting down at the keyboard. Administrators attribute her antics to self-indulgent eccentricity, but it might be asked whether Argerich's day-at-a-time approach makes any less sense than the five-year-plan mentality that prevails in the executive suites. These days, performers are asked to be brilliant onstage and dronelike in transit; they are expected to commit to 11 A.M. rehearsals in the year 2006. The result is a lockstep concert world in which New York débuts feel like the ratification of deals made years ago and celebrity initiatives have the momentum of continental drift. Argerich is driving a wedge into the system, as Sviatoslav Richter did before her, with his spur-of-the-moment piano happenings in towns scattered between the South of France and Siberia.

In the restaurant, hours pass and night falls. I feel that I should go, but it is strangely difficult to leave. The cosmopolitan chatter goes on at a dotty pace, as if we had all been thrown together at some turn-of-the-century spa. Argerich is asked why she did not play the concerto by Alberto Ginastera that had been announced for the Carnegie series. "I did not learn it," she replies. The conductor Christoph Eschenbach drops by. He has just conducted Samuel Barber's "Adagio for Strings" amid the ruins of the World Trade Center. Argerich trembles as he describes the scene, and touches his arm when he mentions the smoke and the smell. The friends remain at the table, and the dinner menus reappear. Eschenbach notes that from where he is staying, opposite Juilliard, he can see students slaving away at pianos until midnight. He mimes an intense young person practicing. "That is what I should be doing!" Argerich exclaims. Everyone laughs.

In the event, Argerich's withering self-critique was not completely at odds with reality. In the first movement of the Tchaikovsky First Concerto, she seemed ill at ease, with the huge marble chords of the introduction sounding a little rushed and jumpy. But the sense of effort—rare to the point of extinction in Argerich's playing—disappeared when she reached the oasis of the slow movement. It was as if she had given up on the idea of a great performance and begun playing only for herself. In the process, a great performance emerged. She rescued Tchaikovsky's warhorse from the usual virtuoso bludgeoning and restored its sense of aristocratic play. It came out sounding like new music, contemporary music, radical tonality.

In the Beethoven First Concerto the next afternoon, Argerich performed a similar feat of transfiguration. Again, it was playing of dazzling lightness, almost jazzy in tone. The glistening runs were marked with tiny jabs of emphasis, about one in a bar, so that they seemed to dance above the ensemble. Then, just as the music was ready to float away to some Ravelian heaven, demonic runs rose up from the bass. The way the piano suddenly thunders under the pressure of Argerich's small frame is a physical fact that resists explanation. It can only be described as a possession, a visitation, such as seems to happen when great singers take the stage.

Argerich's performances reminded me, in an oblique way, of Waltraud Meier's singing of the role of Isolde, which had astounded a Carnegie Hall audience the previous week. The soprano had appeared in a concert performance of "Tristan" with the Chicago Symphony, under Daniel Barenboim. Meier's Isolde, blind with rage at the beginning, became uncannily serene at the end, lost in a world of her own making. On a more intimate scale, in the slow movement of the Beethoven, Argerich offered a similar epiphany—a sense of spacious, encompassing solitude. "Do I alone hear this melody?" Isolde sings. Not anymore. ♦