Perhaps the most exacting single challenge in Rudolf Serkin's repertory is Beethoven's "Thirty-three Variations on a Waltz by Anton Diabelli, Op. 120," which he played at the Kennedy Center Concert Hall on Saturday night.

Lasting almost an hour, the "Diabelli Variations" are longer than anything else in the basic piano literature. They are unprecedented in their daring and complexity -- a climactic encyclopedic review in his last years of the piano's expressive potential by the man who wrote more great music for the instrument than any other. In these sublime distillations of the rhythmic, harmonic and dramatic essences of a banal 32-bar waltz by Diabelli, the Salieri of Beethoven's time, Beethoven was writing primarily for posterity -- specifically addressing himself to the precedent of Bach's slightly less massive "Goldberg Variations."

It may have been just a coincidence that Serkin chose to play the "Diabelli" Saturday night, but what more conclusive way to demonstrate, as he enters his eighties, that his awesome powers are essentially undimmed.

At this point, Serkin brings more to the "Diabelli Variations" than any other artist. Playing as he does, the variations do not so much dazzle as they simply overwhelm.

Serkin attacks them fearlessly. There are few pianists even half his age who have the kind of steady physical stamina that he showed Saturday night. Just getting through the work is an impressive accomplishment. Playing the variations with the intensity and the command of the mechanics of the instrument that Serkin showed is something else again. He brings an almost missionary conviction to the work.

The range of the conception was clear from the first bars. Normally thought of as a rather sober interpreter -- a master of complexity and ambiguity -- Serkin launched into that little waltz with captivating gusto and almost orchestral sonority. His right leg, used sparingly on the pedal, was positively dancing to the music. Then came the march variation, sweeping away the lilt of the waltz and sounding almost like "Meistersinger" the way Serkin played it.

And the music proceeded -- with the most intent seriousness but also full of charm and bite when the occasion struck. Could one imagine more brio in the variation marked "allegro con brio"? And the following one, based on, of all things, Leporello's "Notte e giorno faticar" from Mozart's "Don Giovanni," floated in with a deliciously wacko incongruity.

The three tragic slow segments in C minor near the end unfolded with heart-rending gravity, building tension only to be dispelled by the mighty double fugue that seems to release all the stresses that had built over the hour.

Earlier, Serkin played the brief Op. 78 Sonata and the "Les Adieux" Sonata more pensively than normally heard -- and with great authority.

At the concert's end, the audience -- which overflowed onto the stage -- rose almost as one in tribute to the pianist. There was no shouting, though; it would have been inappropriate.