Fragments, Young Vic, London

Nobody could accuse Samuel Beckett of having a Pollyanna-ish approach to existence. But he's not Eeyore either. He faces up to the terminal – or, rather, not yet terminal – bleakness of life with the courage of pedantically exact and exacting comedy. If you want to know the scale of your misery, he will give you the precise measurements. His palette may be mostly grey and black, but he'll show you the sensitive range through which the colour grey can travel, and throw in the odd splash of banana-skin yellow.

There's vigour in his distrust of vitality. There's a kind of liberating joy in the clarity with which he perceives life as a grim practical joke played on those who are lumbered with it. The idea that there's nothing funnier than unhappiness could, in a lesser writer, be the occasion for callous crowing, but Beckett is also very funny about the poor returns that come from schadenfreude.

All of this is richly evident in Fragments, five short Beckett pieces arranged as a kind of suite by Peter Brook. The show may embrace subjects such as manic depression and suicide, yet it constitutes one of the most life-enhancing evenings in London theatre. Last year, Brook brought the piece over from his base at the Bouffes du Nord in Paris to the studio space at the Young Vic. It now returns to the same theatre's main house, where it benefits from the airier environment – the miniature pitted against a sense of immensity, the strange musical progress from one piece to another intensified by the broader dimensions.

The most serious and profound of the pieces is Rockaby, performed with revelatory power by Kathryn Hunter. In the text, the woman is mechanically rocked in her dead mother's chair while she listens to the recorded sound of her own voice as she proceeds, via rhythmic repetitions of phrase and preoccupation, towards suicide. In Brook's version, Hunter delivers the lines live, and rocks an ordinary chair by hand until the final movement of the monologue.

Astonishingly, Hunter – tiny, intense, glittery-eyed – finds a black comic energy in the woman's lines. It's as if she is reacting to phrases that are, so to speak, talking through her. I was reminded of a great poem by William Empson, "Let it Go": "The talk would talk and go so far aslant./ You don't want madhouse and the whole thing there". Someone is spreading rumours against you, and that someone may be you. Hunter's brilliant performance is deeply suggestive of this. And the end here feels less like suicide than a kind of auto-euthanasia. The woman has so looked upon herself in the third person that it feels more like the product of pity than of despair.

The existential slapstick of Rough for Theatre I and Act Without Words II is hilariously handled by Marcello Magni and Khalifa Natour. The first piece is about a crippled vagrant and a blind beggar who attempt to speed-date so that they can have the use of one another's faculties. In the second, Magni scowls and huffs as if trying to fulfil a daily quota imposed on him by God and his genes, while Natour is as dippily and dogmatically cheerful. The pair are alternately prodded awake from their plastic bag homes by a goad that dangles over them with all the deadpan incipient violence of Rod Hull's Emu.

In the last sketch, Come and Go, the two actors – now in drag – are joined by Hunter for a piece in which three effortfully respectable old biddies who were at school together pair off against the third in a mime of outraged gossip and mounting mutual suspicion that is a masterpiece of comedy acting. There's a wonderful moment where Magni's old crock, who is beginning to see a pattern in their behaviour, hesitates for just a second before consenting to continue it. Thoroughly recommended.

To 13 September (020-7922 2922)