‘Springing like Minerva fully armed from the head of Jove’ was Robert Schumann’s euphoric verdict when the 20-year-old Brahms visited him in Düsseldorf in 1853. One of the works that excited Schumann was ‘Liebestreu’, the Brahmsian equivalent of another early song masterpiece, ‘Gretchen am Spinnrade’, in its mingled technical certainty and expressive power. It makes an apt opener to Sophie Rennert’s recital, the final volume in Graham Johnson’s complete Brahms survey. Supported by the ever-observant, ever-sympathetic Johnson, the young Austrian mezzo responds vividly to each phase of the mother-daughter dialogue, finding an unforced depth of tone for the mother’s words and a limpid pianissimo – ‘träumerisch’ (‘dreamy’) as Brahms requests – for the abandoned girl’s response. With almost imperceptible quickenings of the pulse, vindicating their unusually slow opening, singer and pianist create a sense of ineluctably mounting tension right through to the searing close.

Brahms was always drawn to the chiaroscuro of the mezzo-soprano voice and its instrumental equivalents, the clarinet and the viola. I suspect that Sophie Rennert’s tones, combining an amber glow with the freshness of youth and a free-soaring top, would have come somewhere near his ideal. She phrases broadly and expressively, and points and colours words with unexaggerated sensitivity. Her diction throughout is a model. Rennert balances artlessness and dramatic involvement in the many folk songs and quasi-folk songs (Brahms was a dab hand at inventing pseudo-folk), and is a keen storyteller in the grisly ballads ‘Lied’ and ‘Murrays Ermordung’, two of many welcome rarities here. Amid so many Brahms songs of loss, longing and abandonment, Rennert seizes the moment when the mood brightens: say, in ‘Von waldbekränzter Höhe’, its erotic excitement urgently caught, or the sensuous stillness of ‘Unbewegte laue Luft’, a song for ‘a slimmed-down Brahmsian Isolde’, as Johnson puts it in his probing notes.

Occasionally, as at the openings of ‘Die Mainacht’ and the deathbed song ‘Immer leiser’ – both notorious tests of legato – Rennert swells into individual notes rather than spinning an unblemished line. In both ‘Die Mainacht’ and ‘Von ewiger Liebe’ the initial tempo might seem dangerously slow. ‘Von ewiger Liebe’ opens with an almost funereal tread, bereft of an underlying waltz lilt. But, as in ‘Liebestreu’, each of these great songs unfolds with a sure sense of growth and an illuminating range of colour. ‘Die Mainacht’ emerges as an intensely private, musing soliloquy, the huge lamenting phrase at ‘Und die einsame Träne rinnt’ expressively shaped in a single breath. Gerald Moore once described ‘Von ewiger Liebe’ – perhaps the most moving expression of undying love in the entire lied repertoire – as ‘a big song, and it must be performed in a big way’. It is here. With subtly managed tempo changes, Rennert, mining the coppery depths of her mezzo, and Johnson (superb in the quasi-orchestral climax) catch both the boy’s impetuous unease and the girl’s increasingly impassioned certainty. The hushed intensity of her declaration of faith (‘Eisen und Stahl, man schmiedet sie um’) is a magical moment, truly dolce, as Brahms asks.

Among rival mezzos in this repertoire, Anne Sofie von Otter (DG, 4/91) brings more passion, and a sharper tinge of anxiety, to the two Op 91 songs. But in close communion with the eloquent Lawrence Power, Rennert compels with her inwardness and gently fluid phrasing in these love duets for voice and viola. Unfurling her bold chest tones, von Otter also displays a touch more sheer bravado in the Zigeunerlieder. Yet Rennert and Johnson capture both the gypsy swagger and (above all in the exquisite No 7) the tender melancholy of these songs, with a fine control of rubato that is a hallmark of this whole programme. A lovely singer, Sophie Rennert is a natural in lieder. Her partnership with Johnson makes a fitting culmination to what has proved a revelatory Brahms series.