"It floats like a breath," said the tall and elegant woman with the beautifully bred terrier, spring-loaded and taut on a lead. But, then, after 18 years as director of London's Serpentine Gallery, Julia Peyton-Jones is genetically predisposed to the gracious dispensation of almost irresistible one-liners.

The "breath" she refers to is the new Serpentine Pavilion, designed by the Japanese architects Kazuyo Sejima and Ryue Nishizawa, which opens to the public on Sunday.

But it isn't a breath. Nor is it smoke, or a reflective cloud, or a floating pool of water, or anything else that the Serpentine's wordsmiths have come up with. The architectural quest for metaphors is always problematic, and very often leads to hyperbole. Which would be a pity in the case of Sejima and Nishizawa's pavilion because, in essence, it is simply a very thin, shy, rather beautifully made aluminium wafer wavering three or four metres above the greensward in Hyde Park on gleaming, stick-thin legs. Were it not quite so exquisitely elegant, the pavilion might be something encountered in an historic sci-fi props museum in Hollywood, labelled: "Zargon time machine from scene 12, They Came From Tectonica, 1954, director: Milton Shubasky Jr."

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There is so little of the pavilion that it barely exists. It conveys no sense of mass, weight, enclosure, or dimension. Even the lines of its form are tenuous. The only things that register distinctly are the almost impossibly thin edges of the roof, the sheeny surfaces, and those shimmering, stork-leg posts that hold it up.

It is as if an amoeba-shaped sliver of aluminium had risen delicately in the mind of a graphic designer dreaming of a world-beating organic brandmark for the 2028 Olympic Games; risen, and then began to settle again in a breeze wafting up Exhibition Road before being freeze-framed into something either rising or falling – doesn't matter which – with some of its gloopy edges tipped up, and others caught and held in mid-loll.

The pavilion is an architecture which shies away from architectural reference. Modernist? Not really, because there's no vorsprung durch technik about it. 21st-century Arts and Crafts, perhaps? No, sorry, the ghost of William Morris is unlikely to materialise on Sunday. Well, it must be a computer-generated whatsit, then, conceptually linked to a series of magic prime numbers – except that there's no sense of geometrical obscurity about it. Nor does the pavilion express the kind of left-field geometry that spools out of the architectural imaginations of designers such as Ben van Berkel and Cecil Balmond.

So, how do the architects – aka SANAA – explain their building? The reflective canopy undulates across the site, they say, expanding the park and sky. Its appearance changes according to the weather, allowing it to "melt into the surroundings". "At first," says Nishizawa, "we had no architectural idea. Just water, rainbows, leaves or something. Finally, we reached the organic shape for the roof, to create a relationship with the trees."

And that's the point. The pavilion does not melt into the surroundings. Nor does it, as they also suggest, sit "seamlessly within the natural environment". If it did, it wouldn't be engaging in a meaningful way. Why must architects insist on imagining that buildings fit seamlessly into their surroundings when they themselves, along with the rest of humanity, inert gases, languages, ideas, X-boxes, and loaves and fishes – patently do not?

If the pavilion is architecture – which I think it is – it can only be so if it reacts to its surroundings in something more than a passive, don't-mind-me manner. The pavilion may barely exist, physically, but it has a presence – and that presence is created by the tensions of its form, structure and abstraction.

The tension of its structure, for example. One expects it to tremble in a gust of wind, and would be very disappointed if it did not. And those sharp, silvery roof edges, like etched lines against the surrounding trees, sometimes distinct, sometimes not; the reflections of clouds and sky, the felty greens of the horse chestnut and catalpa trees, smeared and acid-trippy on the pavilion's highly polished aluminium skin.

This is Aphex Twin architecture: not quite ephemeral, subtle in its sensual harmonics, but seamed with unexpected visual and formal riffs. It's going to be popular with the bong brigade at sunset – "How did the sky get so totally in here, man?" – and to those of filmic persuasion. There is something uncanny about the pavilion, something that triggers the unearthly atmosphere of the park scenes in Antonioni's Blow-Up.

SANAA's offering is the ninth pavilion in the Serpentine's annual series, and it's the practice's first completed project in Britain. Ms Peyton-Jones, and her advisers, have conspired to create these Hyde Park surprise packages in an even-handed way. The architects selected have either been the senior magi of the profession, such as Oscar Niemeyer and Alvaro Siza; intellectuals including Rem Koolhaas and Daniel Libeskind; fashionable A-listers like Zaha Hadid, Frank Gehry and Olafur Eliasson; and coolly acronymic new-wavers such as SANAA and MVRDV.

And how does SANAA's architecture stack up against the pavilions that preceded it? Well, it doesn't have the lusciously overstated grace of Niemeyer's, even if it does recall the Brazilian's 1943 dance hall canopy at Pampulha; it doesn't present such an obviously brilliant essay in geometry as the Ito-Balmond pavilion; it is not as brusquely strange as the pavilions by Siza or Gehry; and it is not a literally inflated idea, like Koolhaas's.

SANAA's design is about something else: edges that shy away from defining volumes of space, and surfaces that are equally vague about what is up and what is down, in or out. Not that this is anything new to Sejima and Nishizawa's architecture. Their New York Museum of Contemporary Art resembles a skewed stack of giant ice cubes, not quite solid, not quite porous; and the Kumanokodo Nakahechi Museum in Japan repeats the effect, but in a modernist pavilion whose precision is worthy of Mies van der Rohe or Philip Johnson.

SANAA's position in the 21st-century pantheon of rising stars is therefore already assured, and by the time their new Louvre museum at Lens, France, is completed in 2012, they will be members of architecture's established inner-circle.

One worries about Charlie, though, Ms Peyton-Jones' admirable terrier. What if she has a serious go at one of those skinny aluminium pillars, just to remind us that architecture that pretends to melt into its surroundings needs a damned good savaging?

Serpentine Gallery Pavilion 2009, 12 July to 18 October (020-7402 6075; www.serpentinegallery.org)

Modernist marvels: Four more SANAA success stories

Zollverein School, Essen, Germany

This is a chunk of Japanese urban modernist architecture transplanted into a German setting. The asymmetry of the windows, set into otherwise unremarkable facades, give thebuilding a sense of abstraction and tough playfulness.

Glass Pavilion, Toledo Museum of Art

The pavilion’s interiors are SANAA’s graceful homage to Mies van der Rohe’s Barcelona Pavilion.But the curved glass and warped reflections begin to hint at what they designed at both the Serpentine, andthe forthcoming Louvre annexe at Lens, France.

Dior, Tokyo

SANAA have joined the premier division of architects who have designed buildings for famous fashion houses. The materials and detailing are ultra-rich in every sense, but not as brilliant as in Herzog and de Meuron’s Louis Vuitton building in the same city.

New Museum of Contemporary Art, New York

The architects have, in effect, joggled a pile of giant aluminium boxes to reduce the perceived mass of the museum. But the textured aluminium surface is the real key, because, in a gritty part of the city, it creates the coolest of auras.