Jazz history boasts many saxophonist-vocalist pairings – Stan Getz and Astrud Gilberto, Lester Young and Billie Holliday, to name but a few – and so the alliance between these two jazz moguls, Marsalis and Elling, finds itself in excellent company. The driving idea behind their Grammy-nominated album Upward Spiral – which dominates much of the night’s setlist – was to create a ‘true partnership’ and to highlight Elling’s voice as an instrument, but what’s so striking here is the way Marsalis mirrors this reversal in his own sax playing. In the pooled blue light of the Barbican stage, Elling’s voice becomes instrument and Marsalis’ instrument becomes voice.

Marsalis’ long-running, high-voltage quartet – powered by Joey Calderazzo on piano, Eric Revis on drums and Justin Faulkner on bass – is no house band: the evening gets underway with an explosive performance of ‘Mighty Sword’, penned by Calderazzo himself.  When Elling saunters onto stage – the epitome of Chicago ultra-cool – the quartet do not hold back in order to accommodate him: he’s there as a partner-in-jazz and not a frontman – and the dynamic works a treat.

We’re treated to a single two-hour set of astonishing variety here. ‘There’s a Boat That’s Leaving Soon from New York’ from Gershwin’s ‘Porgy and Bess’ is delivered with a captivating balance of charm and danger, with Elling’s vocals swooping from rakish crooning to hip swagger. ‘Blue Gardenia’ is a masterclass in pensive restraint; Jobim’s ‘Só Tinha de Ser Com Você’ shapeshifts from breezy bossa to something more storming and shuffling altogether. Elling and Marsalis are each other’s perfect complements – both literally and figuratively stepping backward into the stage’s shadows to accommodate each other’s melodic lines. This was in best evidence on the mesmerising study of Sting’s ‘Practical Arrangement’. I’d never realised quite how supremely depressing the lyrics are (‘It may not be the romance that you had in mind/ But you could learn to love me, given time’) but in Elling’s mouth, the words are stretched out into an unexpected note of hopefulness, twinned marvellously with Marsalis’ luxuriantly melancholic sax.

The highlight of the evening, however, was the first encore – a spare duet between Marsalis and Elling on ‘I’m a Fool to Want You’ – where voice and saxophone became spellbindingly indistinguishable.