Matt Damon's latest movie a flop?

Occupying a curtain-raising slot that has come to be seen as a launchpad for films with Oscar ambitions, Alexander Payne's part satirical, part save-the-planet new work was hailed as a breath of fresh air from the "Sideways" and "Nebraska" director.

The Hollywood Reporter said Payne had "hit the creative jackpot", while Variety welcomed a "ticklish and resonant crowd pleaser for grown ups".

London's Evening Standard was more reserved, praising the film as "often very funny" but bemoaning the abandonment of its initial satirical edge.

Set in the near future, the film is based on the premise that scientists have found a way to literally reduce humanity's environmental footprint by downsizing humans to five-inch (12.5-centimetre) versions of themselves.

But it's not long before the technological breakthrough is exploited for different reasons, enabling people to access a much more luxurious lifestyle than they could ever afford in the big world.

A combination of ecological and material motivations for being shrunk appeal to Paul Safranek (Damon), a frustrated but well-meaning therapist, and his wife Audrey, played by Kristen Wiig.

They sign up for the surgery but she gets cold feet at the last minute, leaving Damon to embark alone on his adventure in the miniaturised world.

Among those he meets there are Christoph Waltz, who plays a louche, party-loving neighbour in his miniature condominium, and Vietnamese cleaner Ngoc Lan, played by Hong Chau.

She, it turns out, had been forcibly miniaturised after being imprisoned as a dissident in her homeland and has lost a leg below the knee as a result of being smuggled into the United States in a television box.

Damon strikes up a friendship with her, taking the film in an unexpected Romcom direction that allows Payne to tie up his themes about the search for a better life, impending environmental catastrophe and the need to live in the moment.

"It's like a journalist I was speaking to said: it is Alexander's most optimistic movie and it's got the Apocalypse in it," Damon quipped at the premiere press conference.

Scripted by two-time Oscar winner Payne and his frequent writing partner Jim Taylor, the film will be hoping to emulate the success of "La La Land", "Birdman" and "Gravity", all Venice openers which went on to bag awards.

Taylor said the film had been in the works for over a decade, playing down suggestions it was a commentary on Donald Trump's controversial environmental policy.

"A lot of things caught up with the movie, we didn't realise we were going to be sharing the film with the world we are living in."

Also unveiled on the opening day was "Nico", a bio-pic focusing on the final years of the Velvet Underground singer and Andy Warhol muse, shown in the festival's "Horizons" section dedicated to cutting-edge productions.

- Redford and Fonda -

"Downsizing" is one of 21 films competing for Venice's top prize, the Golden Lion, which will be handed out on September 9 along with other awards including the first for virtual reality productions.

A total of 71 films on show range from big-budget Hollywood productions, like George Clooney's sixth directorial outing, "Suburbicon", to new works by indie favourites Andrew Haigh and Warwick Thornton, via documentaries such as Chinese artist Ai Weiwei's look at the global refugee crisis, "Human Flow".

British director Haigh will be presenting "Lean on Pete", his first film since the acclaimed "45 Years", while Thornton arrives in Venice next week to promote "Sweet Country", a Western set in 1920s Australia that deals with the treatment of indigenous people.

Along with Clooney, major stars due on the red carpet include Robert Redford and Jane Fonda, who will pick up lifetime achievement awards while plugging their new film "Our Souls at Night", a Netflix drama about a romance between two elderly neighbours.

- Bloodied heart -

Spanish superstar couple Javier Bardem and Penelope Cruz team up again for "Loving Pablo", in which Bardem plays Colombian drug baron Pablo Escobar and Cruz his long-term mistress.

Bardem also stars opposite Jennifer Lawrence in "mother!", one of several thrillers vying for honours, by "Black Swan" director Darren Aronofsky.

Promoted by a Mother's Day-release of a poster showing Lawrence holding her own bloodied heart, the film tells the tale of a couple thrown into turmoil by uninvited guests.

Also expected to make waves, with an out-of-competition world premiere, is "Victoria & Abdul", Stephen Frears' treatment of Queen Victoria's later-life friendship with an Indian clerk.