‘I like to create a front room feel,’ says Niki King of her relaxed, sometimes sultry, always silky smooth, live gigs. The Edinburgh jazz singer won the Perrier Jazz Vocalist Award in 2001 and has since released four albums, opened for Al Green and been given a shiny seal of approval by Radio 1 DJ Gilles Peterson, who invited her to perform on his album, Para:Diso. For her return to the Jazz Fest, she’s bringing her long-time collaborator, the guitarist Marcus Ford. ‘We’ll have only two instruments onstage,’ says King. ‘His guitar and my voice, both celebrating beautiful, timeless songs. It’s slightly more of a challenge than performing with a full band, and you’ve got to give it 150%. But that’s how we like it, so there’s nowhere for us to hide.’

In the past, the ‘front room’ atmosphere has meant the crowd gets to share tiny footnotes on King’s love life; in between songs she’ll slip in a confession about a messed-up first date or a painful break-up, or watch while Ford restrings his guitar after an untimely ‘ping’ during the set. But for King, it all just adds to the experience. ‘Marcus’ string snapped when we played The Hub last year, which wasn’t ideal. But I guess it reminds you how unpredictable a live gig is. I love that; it’s the difference between listening to a pre-recorded album and sharing something raw and real with a performer right in front of you.’

In the early days of her career, King was lead vocalist at the now defunct Café Graffiti’s Lizzard Lounge in Edinburgh, where she flipped between trip hop and soul styles, and more recently she has performed with her seven-piece band. They joined her onstage at The Jazz Café in London for the launch of her 2004 EP ‘Letting Go’, then in the Radio 2 studios when she sang on Mica Paris’ show, Soul Solutions. But King admits she’s grown to love the intimacy of her set-up with Ford. It’s a pairing where both feel free to improvise, dovetailing his gentle acoustic strumming with her bossa nova sassiness or heartbroken longing. ‘We’ve worked together for so long now, we trust each other completely,’ says King of their partnership. ‘On stage, if he wants to go off on a solo, I’ll give him space, and if I want to play around with the melody a little, he’ll follow me. It just works.’

As well as performing live at concerts in Germany, Italy, Japan and China, the duo recorded the album Twelve O’ Clock Tales in 2006. It was a departure from King’s own self-penned songs, and she chose to cover a cherry-picked collection of her most loved tracks, including one of her all-time favourites, ‘Until it’s Time for You to Go’, the bittersweet love song by Canadian folk songwriter Buffy Sainte-Marie, and a feather light, open-heart re-working of Elvis Presley’s ‘Always on My Mind’

‘We first decided to add in the Elvis song about half an hour before a gig,’ reveals King. ‘I learnt the lyrics backstage in about 20 minutes, but it seemed to go down really well with the audience, so we ended up putting it on the album.’ When picking their set list, King mixes up whispery jazz standards and classic songwriting, letting her voice float and soar around songs by Stevie Wonder, Thelonius Monk, Jacques Brel or Tom Waits. ‘The era isn’t important to me,’ she explains. ‘My job is to get across the story, and the power of the song.’

Although their repertoire is wide - straddling funk, soul, jazz or country genres - each song seems to end up with a very audible King stamp of quality on it. ‘Covering a song is all about making it your own, adding something fresh and making it memorable,’ says King, whose own make-overs usually blend moments of ear-stroking tenderness with octave climbing bursts of intensity. ‘If Marcus and I are relaxed and confident up there, then the audience feels comfortable too. When you get to share a room with a bunch of people who seem like they’re really into these songs too, that’s a real joy.’

Niki King and Marcus Ford, The Hub, Royal Mile, 0131 473 2000, 31 Jul, 10pm, £10-£12.