Cat Power With The Memphis Rhythm Band, Barbican, London <!-- none onestar twostar threestar fivestar -->

It may be tough to believe, but Cat Power, aka Chan Marshall, makes a heroic stage entrance tonight. Backed by a 12-strong band of session musicians famed for working with soul greats such as Al Green, the alt-indie star with the parched, haunting blues-folk voice dances, skips, strikes muscle poses and mock-moonwalks as the title track from her latest album, The Greatest, unfurls luxuriously. As surprises go, it's a little like watching Richard Ashcroft discover understatement or Nick Cave go disco. Pinch yourself, and she's still up there, looking largely happy to be so.

Notoriously, Marshall's concerts can be choppy. It's tough to tell if this is down to a wilful dismantling of performance clichés or sheer raw nerves, but on an off night, the effect can range from frustrating to discomfiting. Why perform, if it's so painful? Equally, why watch someone struggle so much? When the tour to promote The Greatest was initially cancelled because of an unspecified illness earlier this year, it was hard not to think of the breakdown that laid Marshall low in 1998, prompting her to record what she called the "salvation songs" of her Moon Pix album.

But her current, rescheduled shows have been hinting at another salvation, of sorts. Played in order, The Greatest makes up the bulk of the set, with just one song dropped ("After It All") and a lyric changed. The lyrics hinge on the push-pull of aspiring to confidence and the undertow of self-hatred, the title track alone veering from "Once I wanted to be the greatest" to "And then came the rush of the flood...". On "Islands", Marshall sings of wanting to rule over islands and sea, only to add, "But if you're not coming back/ I will sleep eternally." On "Lived in Bars" tonight, she mimes cutting her throat with an index finger to lyrics about "ending it all".

Later, though, there's that aforementioned lyric change. Lent emphasis by Marshall playing it raw and acoustic, the words on "Hate" shift from "I hate myself and I want to die" to "I do not hate myself and I do not want to die". During that same acoustic interlude, she declares herself "sober now", albeit hooked on tea. At any rate, something seems to have been overcome during her recovery time.

Her unusual collaboration with the Memphis Rhythm Band makes sense in the context of this push-pull between despair and resolution. The result maps out a tension between the sparse, deep-rooted, introverted Americana of her delivery and the band's optimistic soul clout, and it emphatically works live. As Marshall weaves around the mic-stand like a prize-fighter, tugging at her shirt-sleeves edgily (that push-pull again), the full-band backing ranges from sympathetic to near-liberating.

When Marshall first arrives on stage after an instrumental warm-up for the band, the volume seems to dip ever so subtly, as if attuning itself to the way her hushed voice creates space around itself. When she sings "give them some air" on "Lived in Bars", the backing stretches and yearns fittingly, giving Marshall breathing-room. As the same song's tipsy languor breaks into a strut near its close, Marshall visibly loosens up in the music, nervous fidgeting put to one side as she sings with a discreet sense of triumph about flying "out of here".

Granted, a wholly tension-free Cat Power concert is an unlikely thing, and some old frustrations do return for her mid-set solo spot. But the nerves don't show on an encore of "I Don't Blame You", as Marshall sits at the piano and sings with lucid empathy about a musician's struggles to perform. Likewise, on the set-closer, "Love & Communication", the band provide big, beefy backing for Marshall to rise to on lyrics such as "The good part was/ That I came at all". That was good, indeed, but there was near-greatness, too, in tonight's articulations of suffering and tentative redemption.