In his eighties, Rudolf Serkin has apparently decided to play nothing but masterpieces. He has been training for this kind of assignment throughout a career that began in 1915. And as the years accelerate, nothing less than the best deserves his attention.

Serkin is hardly relaxing at 82. He declined to give any encores after his program, yesterday afternoon at the Kennedy Center, but the program itself was more than a full day's work for any pianist. And he played it with a polished, assured technique that might be envied by men a half or a third his age.

There were three masterpieces on the program -- four if you divide Mozart's Fantasia in C minor, K. 475, from his Sonata in the same key, K. 457. The audience separated them by applauding with well-justified enthusiasm at the end of the Fantasia. Serkin linked them by launching into the Sonata as soon as the applause would permit. So did Mozart in 1785 by publishing the two works together. Whether you call them one work or two, they validly belong together, linked by an intricate array of similarities and contrasts.

The Fantasia sets the tone of the linked pair in an adventure of the spirit that glances back briefly toward Bach and adventurously ahead toward Chopin. Its free form gives the composer and performer (who were originally the same) unusual scope both for virtuosity and for adventurous stylistic excursions into the regions of Romanticism. Energy, wide-ranging in the Fantasia, is channeled more formally in its nonidentical twin Sonata; it is not energy in the acrobatic 19th-century sense, expressed largely in speed and loudness, but an energy of internal pressure, working through rather restrictive forms and heightened by the restrictions.

Serkin's interpretation gave the impression that the composer and performer were, once again, the same person -- as they had been in the 1780s. Hearing it, one was impressed not with the abilities of the pianist (though they were abundant and spectacular), not with the distinctiveness of his interpretation (though it was there), but with the quality of the music. Serkin became a transparent medium for Mozart. In a sense, for the duration of the music, he was Mozart. Then he became Beethoven for the "Waldstein" Sonata and Schubert for the great, posthumous Sonata in A, D. 959.

The Schubert, a powerful blend of sometimes wayward lyricism and intense drama, was presented with clear, penetrating vision. But the climax of the program was the "Waldstein," a musical statement whose greatness and complexity transcend the limits of any single performance. Serkin's interpretation was not quite perfect, if one looks for microscopic technical lapses, but it had a quality that went beyond mere technical perfection. It was pure poetry, a dazzling exploration of the music's depths.