BASEL, Switzerland - The global art market appeared to collectively sigh with relief as deep-pocketed collectors descended on Art Basel this week after two years of dwindling sales.

The world's biggest contemporary art fair opens to the public on Thursday, but VIPs got an advance peak at the vast array of artworks for sale.

They range from 20th century masters like Pablo Picasso to today's cutting-edge creations.

Nearly 300 galleries representing more than 4,000 artists from around the world have put their best goods on display at the show, which has become unmissable for sellers and collectors alike.

"The mood is very, very strong," enthuses Art Basel director Marc Spiegler.

"There are great collectors here. Great artworks. There is a very good energy. Very good atmosphere," he said ahead of the public opening, adding that "sales are being made".

That is good news for the global art market, which in 2016 was valued at $56.6 billion - down 11 percent from a year earlier, according to a study by Swiss banking giant UBS, the fair's organizers.

It was down a full 17 percent compared to 2014, when the global art market reached its pinnacle value of $68.2 billion, before geopolitical turbulence put the breaks on investors' ebullience.

As a sign the pendulum may be swinging back in their favor, 116 private jets were expected at Basel airport on Tuesday when rich collectors descended on the show - 18 more than last year.

Galleries boasted numerous large sales in the first hours after their booths opened.

"We did a few sales around a million dollars," says Mathias Rastorfer, head of the Gallery Gmurzynska. Works by Fernand Leger, Wilfredo Lam, and Roberto Matta were among those that found new homes, he says.

Marc Glimcher, president of Pace Gallery, agrees that "Art Basel is fantastic this year."

His gallery sold about a quarter of its booth within the first hour, he says, mainly pocketing checks in the $100,000 to $800,000 range.

And he says most of the multimillion-dollar pieces were already on reserve for museums, while he had numerous collectors vying for a $6.5-million David Hockney.

Brett Gorvy of the new Levy Gorvy gallery also hailed the "fantastic energy" at the fair. Works were selling "very, very quickly", including an Alberto Burri piece for $4.5 million.

The gallery also had Jean-Michel Basquiat's painting Baby Boom on reserve, the asking price $32 million.

But while the high-end galleries and the top artists are raking in sales, they acknowledge that the entire market is not faring as well.

"The top end, if you want to say the bluechips, have become so strong that it has in some ways disadvantaged the middle market," says Andreas Leventis, associate director at Lisson Gallery in London.

This is because mid-level artists are "neither a sure thing or just affordable enough to be a viable risk, from a purely investment point of view," he says.

Rastorfer agrees that there have been "some corrections in the contemporary middle section ... I think they are having a harder time."

When the market is booming and "everything is going up, you don't need much expertise. It just goes up," he says.

But when times are a bit more challenging, "you go to a gallery like us, because we focus on expertise and knowledge," he says.

Agence France - Presse