Witnessing the decline of a great artist can be a distressing experience. The Bulgarian soprano Anna Tomowa-Sintow, now in her early 60s, was, until recently, an outstanding singer, a cult diva with a voice of blazing sexual power, capable of turning in unforgettable performances, not least her Prince Igor, Tosca and Ariadne auf Naxos at Covent Garden.

Two years ago, however, her appearances with the Royal Opera as the Marschallin in Strauss's Der Rosenkavalier revealed that her extraordinary voice had lost some of its sheen. This Wigmore Hall recital with pianist Jean Lemaire exposed a further slippage. The once opulent tone has darkened and become hard-edged. Her high notes, which used to go on forever and send shivers down your spine, can be metallic and are sometimes cut short in full flight.

The abandoned headiness of her singing is now very much in abeyance, though we caught snatches of it in Wagner's Wesendonk Lieder, where the faded voluptuousness of Tomowa-Sintow's sound and her passionate delivery of the text captured the mood of enervated decadence that floods the music.

Her performance of Brahms's Immer leiser wird mein Schlummer was uneasy and uncomfortable; at least the folk-song based Vergebliches Ständchen and Sonntag revealed that her sense of humour is unimpaired. The rapturous flood of sound she once unleashed in Strauss's Cäcilie was only intermittent on this occasion, though she intoned Morgen with a catch in her voice that proved profoundly moving.

As always, her selection of encores was generous, including one of her well-known party pieces, a Bulgarian folk song for which she plays the accompaniment herself with gleeful relish.

The final item was an aria from Cilea's Adriana Lecouvreur, in which the heroine states that she is the "humble handmaid of the creative genius". When it was over, those who remained applauded wildly, though in memory, perhaps, of what she once was rather than the shadow of her former self that she has now become.