I Don't Want to Sleep Alone (15)<br/> The Wayward Cloud (15)

Among the great film-makers, there's eccentricity and then there's eccentricity, but there's a level of strangeness peculiar to Taiwan's Tsai Ming-Liang. As well as being the Asian screen's most eloquent commentator on solitude and city life, he is cinema's bard of bad plumbing. What bureaucracy is to Kafka, faulty waterworks are to Tsai: the indicator of all that's amiss with the human condition. His characters live in apartments prone to flooding, endure torrential downpours, get sick from river pollution. Tsai's latest film, I Don't Want to Sleep Alone, is set partly in a cavernous building site, in the middle of which a vast lake has formed. Perversely, though, its predecessor, The Wayward Cloud (2003), is set during a drought, causing the citizens of Taipei to seek sustenance (liquid and sexual) from watermelons.

At first sight, Tsai's work might come across as "slow cinema" in the extreme: his characters are often seen dragging themselves through life in shorts and flip-flops, not so much world weary as just plain exhausted. Gradually, though, a slow-burn comedy emerges: think of Jacques Tati's visual gags played out on an almost subliminal level.

Tsai's humour can also be broad in the extreme, but even at its wildest, it is consistently muted by regular actor Lee Kang-Sheng, the director's melancholy Everyman. Early on, Lee was well groomed and wanly pretty: over a decade and a half, his face has become more comically blank and babyish. Now the actor, who barely utters a word, has become as distinctive a tragicomic innocent as Stan Laurel, notwithstanding his character's streak of worldly amorality.

While Tsai's films are essentially variations on a theme, there's a huge contrast between his two latest, released to coincide with a retrospective at London's BFI Southbank. One is subtly magical, the other verges on crass. The latter, The Wayward Cloud, is Tsai's loopiest film, brilliant in parts, tedious and a little repellent in others. Lee Kang-Sheng's regular character Hsiao-Kang is now working as a porn actor: he's first seen performing a ludicrous, implicitly gynaecological routine with a Japanese actress and a watermelon. Tsai is nothing if not logical in following the implications of this weird fruit fetish: because of the water shortage, Kang is left sticky from melon juice, swatting away the flies.

The real theme, as ever, is loneliness and the need for human warmth, but Tsai plays the rudiments of a love story as grotesque comedy. A young woman, Chyi (another Tsai regular, the lanky, pensive Chen Siang-Chyi), strikes up a romance with Kang; their relationship comes to a head when Chyi discovers him hard at work with an unconscious actress, in cinema's most bizarre, not to say downright unpleasant, romantic consummation scene.

The Wayward Cloud is interspersed with lip-synched musical interludes, notably a Busby Berkeley routine with Kang dressed as a giant penis. Such malarkey was startling and fresh in Tsai's The Hole (1998); it now looks facetious, more cheap than cheerful. Ultimately, The Wayward Cloud leaves a nasty taste, and not just stale watermelon: the climax is callous, misogynistic and utterly despairing about relationships. But you've never seen anything like it: it's the sort of bad film that only geniuses make.

There's less comedy, more outright poetry in I Don't Want to Sleep Alone (a title that could belong to any of his films), Tsai's first film in his native Malaysia. Lee Kang-Sheng plays two roles: a young man in a coma and a newcomer to Kuala Lumpur (Hsiao-Kang again, to all intents and purposes), beaten unconscious after falling foul of a street scam. He's rescued by a group of men salvaging a moth-eaten mattress, on which he is nursed back to health by Rawang (Norman Atun), a construction worker. Back on his feet. Lee starts an affair with Chyi, provoking Rawang's fury – not only because of sexual jealousy, but also because Kang has made off with his precious mattress.

IDWTSA is quintessential Tsai Ming-Liang in its tender polysexuality, in its obliqueness and minimal dialogue, and in its utterly distinctive use of space: long artificial diagonals of corridors and alleys, and in the building site, Piranesi prospects of staircases and plunging abysses. There are images of breathtaking eerie beauty: Kang fishing in the indoor lake, a huge moth perched on his shoulder; a cluster of fibre-optic lamps on a pavement, glowing like strange submarine flora. This is Tsai's most political film – a story of have-nots in a multiethnic city – and also his most compassionate one, perhaps as a corrective to the harshness of The Wayward Cloud.

One of the most irreducibly left-field of contemporary film-makers, Tsai Ming-Liang may be the auteur's auteur – but if you've ever sweated out a city summer night, hopelessly suffered the hots for someone, or simply had drainage problems, you'll know where he's coming from.

Further viewing Tsai Ming-Liang retrospective is at BFI Southbank till 30 Nov, 020 7928 3232 or go to bfi.org.uk