Rudolf Serkin's program last night in the Kennedy Center Concert Hall started late. "The contract said 7:30," explained Patrick Hayes, coming out on stage 15 minutes after the appointed time, "but Mr. Serkin thought it was supposed to be at 8:15."

That was the last mistake of any consequence made by Serkin -- though, for most pianists, the program itself, consisting of Beethoven's last three piano sonatas, might have been a serious mistake. This is music to test a pianist to his limits. Technically, the test lies in the shaping and articulation of phrases, the balancing of voices, the matching of style, pace and emphasis to the music's rapidly and subtly shifting moods. Spiritually, the test is even harder: the probing of the music's many statements, their meanings and their interlocking overtones.

Like the late string quartets, the other major venture into the strange, brooding, ecstatic world of Beethoven's last years, the late sonatas can be read as a confrontation not only with the end of life but, more generally, to its totality -- a coming to terms with the facts of human frailty but also with the richness of human experience. A virtuoso technique is taken for granted in this music, but it is never flaunted or exploited for its own sake as it is in other sonatas, even as late as the "Hammerklavier." This is music stripped to its essentials--sometimes stormy, sometimes wistful and pensive, always abstractly beautiful but never existing purely in the abstract; music that pulses with life precisely because it dwells on the border between life and death, because it looks back from an autumnal perspective, embodying an all-embracing love of life and a resignation in the knowledge of its limits.

No pianist alive has explored the depths of this music more fully than Rudolf Serkin; none has mastered its technical intricacies so fully. On the eve of his 80th birthday, Serkin's fingers had all the power and dexterity of youth, tempered by the insight and control of long experience. Details could be cited endlessly:

The great cascades of sound in the later variations of Opus 109--slowly, inexorably building to a massive climax before fading away into a simple, peaceful ending.

The pearly articulation of each note in the light, soft, descending scales a dozen measures into the first movement of Opus 130; the exquisite cantabile phrasing of the recitative and arioso dolente, like an impossibly pure and controlled operatic voice; the clarity of the interlocking lines and the precise, moderate pace in the superb fugue that concludes this sonata.

The total fluency, the subtle dynamic gradations of the vigorous opening statement of Opus 111, and the subtlety with which a shout of anger is suddenly transformed into a sigh of regret.

But what emerged from these performances, finally, was not the power and polish of a particular passage or a carefully worked-out detail; it was the total vision, the integrated statement of each movement, each sonata and, finally, of all three sonatas as a single, continuous and completely coherent work of art. It may be all right to play one of these works by itself in a miscellaneous program, as one might play Mozart's "Turkish" Rondo or Litolff's Scherzo without playing the whole work in which they occur. But these three sonatas flow into one another as naturally as do the individual movements within each of the sonatas. At least they do when we have a Serkin to make them flow.

Last night's recital was the most complete and satisfying Beethoven experience this year in a city that has had many Beethoven experiences, including many good ones. It most resembled, perhaps, the cycle of string quartets performed by the Juilliard Quartet at the Library of Congress, with an intermission for the summer. The Juilliard reached similar depths, but Serkin was more finished technically, and the experience was more intense, being compressed into a single evening.

The two experiences are more complementary than comparable, and together they add up to a well-rounded exploration of a territory that can never be completely explored: late Beethoven. There is a striking contrast in the spectacle of a single person, alone on the vast Concert Hall stage with a piano, and four men with stringed instruments communing in the more intimate space of the Coolidge Auditorium. In the sonatas, Beethoven stands alone facing the world, puzzled and sometimes angered but embracing it, reconciled to it. In the quartets, he evokes a small, self-contained society, collectively facing the questions that the sonatas confront in solitude.

Was there anything wrong with the evening? Yes; the piano keys sometimes produced an audible click that was disturbing in soft passages, particularly toward the end of the last sonata, and the audience should have paused slightly longer at the end before breaking the spell with a well-earned, tumultuous ovation. The audience's impatience was understandable; the extraneous piano noises were not.