Brad Mehldau and Chris Thile, Wigmore Hall, London

It’s a strange thing, but the more Brad Mehldau moves away from the standard jazz repertoire, the more imperious and jazz-like the pianist’s improvisations become. He has long drawn from rock’s left field and classical music – Radiohead and Bach were both performed tonight – but this duet, with mandolin virtuoso Chris Thile, pushed the boundaries somewhat further.

Thile is steeped in bluegrass, but also crosses genres and, like accordionist Richard Galliano, extends the range of an instrument often sidelined to whimsy. Thile opened with a traditional bluegrass yodel – the second line “I’m reading my Bible” captured the vibe – but soon decorated his self-accompaniment with squeezed-in lines, shimmers of notes and dark minor chords that pulled at the pulse.

Thile emphasises the guitar-like qualities of his instrument – ideal for the middle three movements of Bach’s Partita in D Minor that followed – but added a banjo-like twang for a brief, breathtaking and raucous display of traditional virtuosity themed, he said, “on the inherent tastiness of rabbits”.

Thile and Mehldau have rarely played together – one song at an Obama fundraiser and a three-number jam before a brief pre-gig rehearsal – and the simple opening theme of “Don’t Be Sad”, played by composer Mehldau, was an ideal consolidatory step. Thile added a breath of mandolin – he plays unusually quietly – they swapped lines, overlaid textures and followed each other’s pathways before mandolin restated the original motif.

Mehldau’s compositions have an optimistic streak –“Old West” came later – but Thile’s are raw with misplaced friendships and indie angst. “Me and Us” and “Alex” mixed gentle strums with raw chords but the remarkably free improvisations burst through diatonic borders into jagged full-length arpeggios and off-pulse stabs delivered at an achingly slow tempo.

Mehldau scrutinises Americana with the same facility and poise that Oscar Peterson and Bill Evans brought to the American Songbook. An early “Don’t Think Twice, It’s Allright” gained harmonic subtleties and showcase breaks, and by the transfixing finale, Elliott Smith’s “Tomorrow Tomorrow” and Radiohead’s “Knives Out” the now intimate duo intertwined freely while exploring deep harmonic waters or, as on The Who’s “We’re Not Gonna Take It” encore, swapped slashed chords and jangles.

wigmore-hall.org.uk