Mitsuko Uchida’s recitals are always a highlight of the New York season, and this was no exception. Superbly ambiguous Schubert, music truly suited for our times, set up a deliciously humorous, characterful, and persuasive account of Beethoven’s Diabelli Variations.

For some, Schubert’s Piano Sonata no. 18 in G major is the placid exception to the anxious, at times violent tendencies of this composer’s later work. Certainly this is not the world of the last string quartet, a destructive work also in G major and completed only a few months before this sonata. And its generally sunny disposition, singing lines, and scattered dances all suggest something more light-hearted than the final trio of sonatas to come. Uchida’s great achievement here was to undermine that, not totally, for Schubert never gives final answers, but just enough.

She managed it through an extraordinary tension between the moment and the overall arc, the vertical harmonies at any given point and the horizontal development of the sonata as a whole. Harmonies here, as in Schubert’s later works, are never quite what they seem, and in Uchida’s hands the major and the minor were far more complicated than in the happy/sad dichotomy we were all taught at school. Indeed, the distinction was often reversed.

The radiant first chord – a sparse, hollowed-out recollection of the opening of Beethoven’s fourth piano concerto – quickly gave way to a tiny disruptions, especially in the bass line and through myriad subtleties in staccato touch. Troubles remained veiled, though, more so than in the Schubert of, say, Paul Lewis, even into the opening of the development, firm, never overdone. There was regret here, reminiscence, the furies of the minor-key outpourings dissipating into an acceptance of sorts in the space of a couple of immaculately voiced chords. Any resolution, by the return to the opening material and indeed later on, was tenuous at best.

Surely, the delightful lilt and airy song of the slow movement asked, there was more joy to be had? Again, not so, as Uchida demonstrated with raging attack in its second theme, and a frankly scary, at least nervous use of the soft pedal to make twirls up high on the keyboard sound disembodied. It ended with a gorgeous smile, after all that. The menuetto was never allowed to settle, its themes contrasting swaggers and smirks, tiny appoggiaturas destabilising everything subtly, and even the trio was never quite content with its rocking self. Could the warm beginnings of the finale be maintained? Almost, although the suspense Uchida kept within and between sections of this rondo always left hope. This was a harmonic working out, as much as anything, rosy in parts and pretty nasty in others. Its final resolution, such as it was, came unexpectedly, a castaway cadence surprising in how suddenly it appeared, and left. This was persuasive precisely because it was unpersuasive.

Perhaps the Beethoven was even finer, a few more slips of dexterity aside, but there is no need in this case to make that kind of claim. Variations these may be, but Uchida reminded us that this is very much late Beethoven. The individual variations, connected of course, often emerged as miniature bagatelles, with all the modernist potential that that implies. They were humorous, too, although as Uchida knows just as much as Alfred Brendel, jokes can be deadly serious. So while Diabelli’s theme was jocular, light, perhaps even naïve, then 9th and 10th variations were only thoughtfully amusing, the Mozart reference in the 22nd was knowing rather than underlined, and the slapstick of the 27th and 28th concealed a gravity of purpose. There was lightness, to be sure, especially in the sunshine of the third variation, but there was also a true violence, particularly in the 14th variation (often all too nice) and the almost mechanised quality Uchida found in the penultimate, fugal section.

Flexibility enhanced rather than dissipated tension, nowhere more so than in the 18th and 24th parts, and that in turn allowed those harrowing slow variations to work their magic in a rare way. Time appeared to freeze in a shock of simplicity, which, as the 31st variation showed, really masked a deep complexity. Returns to Bach meant nothing if not a projection into the future. Yet this is of course Beethoven, and Beethoven has his goals. The cataclysmic climax of that vast fugue gave way to a simply perfect release, as tension dissolved in a lingering final variation, truly a celebration of (and guide to) the human spirit on a par with, although quite different to, the final sonatas, the final symphony, and the Missa Solemnis. Sublime.