Daniel Barenboim/Beethoven Cycle, Royal Festival Hall, London

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It was, as expected, a very public audience with Daniel Barenboim. For those seated on the platform, the ovations were received on an almost one-to-one basis as Barenboim walked among us, reached out to us, like a biblical superstar. Well, this much-anticipated Beethoven sonata cycle was billed as "The Concept of Artist as Leader" – and that he most certainly is.

But for eight heroic concerts he is back to where he began – one man and his Steinway – only this time (I am reliably informed) it really is his Steinway. The piano has been a constant companion on Barenboim's extraordinary journey, and these sonatas will have provided a unique source of sustenance on that journey.

But it was tension of rediscovery and perpetual surprise that made these performances so immediate, so alive. Even where the music seemed to get away from Barenboim's fingers or prove too volatile to be governed by them, its purpose and vision was palpable.

So the very first sonata in F minor quickly shrugged off its dedication to Haydn and rashly took off down its own uncertain path. Barenboim's articulation was freer and weightier, his textures thicker than we sometimes experience. And what was always significant here was the way in which the harmony related to the sound; the way, at the close of the first movement, a single semitonal clash could alter the atmosphere in the room.

These performances flaunted the danger of unpredictability. Sonata No 18 in E-flat had a devilish glint, the seemingly innocuous turning ominous or, conversely, the second subject of the first movement lifting and brightening the demeanour of the piece so that Barenboim felt compelled to turn his head and share the moment with us.

When it came to the mighty "Hammerklavier" Sonata No 29, Op 106, all bets were off. Barenboim had a fight on his hands: the weight of expectation versus the wisdom of experience. All the sorrow in the universe came to find solace in the great slow movement. Barenboim searched for, and found, rare beauty. The central reverie was revelatory, opening magic casements on to a finale that seemed almost to be playing Barenboim, not the other way around. His assault on the climax was so contorted with rage that the familiar was made shocking. This music might have been written next week.