They made their lives in London and were friends for half a century: Francis Bacon, the carousing painter of a nightmare world, and Lucian Freud, a more reserved portraitist.

Now, 16 years after Bacon's death, the pair have conquered the global art world. Last night Bacon's Triptych, 1976, said to be the most important work by the artist still in private hands, was the star attraction at Sotheby's contemporary art sale in New York. Its estimated price of $70 million (£36 million) was only $2 million below the record for a contemporary work at auction, set by a Mark Rothko painting last May.

On Tuesday Freud, 85, became the world's most expensive living artist at auction when his work Benefits Supervisor Sleeping, a lifesized depiction of a voluptuous civil servant lying naked on a battered couch, sold for $33.6 million at Christie's equivalent sale. Freud thus joins the pantheon of the most collectable postwar artists, eclipsed only by Rothko, Bacon and Andy Warhol at auction.

Higher private sales are also claimed for works by Jackson Pollock, Willem de Kooning, Jasper Johns and, in the case of that mysterious diamond-encrusted skull, Damien Hirst.

Pilar Ordovas, of Christie's in London, said that in recent years there had been an extraordinary surge in demand for the best postwar artists on both sides of the Atlantic. “Freud and Bacon were part of an incredibly exciting London scene in the 1950s and 1960s along with David Hockney, Frank Auerbach and Michael Andrews and, although it has taken a while, the marketplace for their work has been transformed. It's the same process with Warhol and Rothko.

“Collectors have realised that you can still buy the best of their work, whereas the best Monets and Picassos are mostly in museums.”

As an artist's prices rise, so collectors bring better works by them to market, Ms Ordovas said. This helps to explain Freud's leap from a record of £4.15 million for Red Haired Man on a Chair, set in 2005, to his pre-eminence at four times that.

Similarly, Tobias Meyer, of Sotheby's, has described Triptych, 1976 as “undoubtedly the most important Bacon in private hands . . . a masterpiece of the 20th century”. The previous record for a work by Francis Bacon at auction was $52.7 million for Study for Innocent X.

Ms Ordovas said of Freud: “He pays great attention to detail and produces not more than four or five works a year. So Freuds are incredibly rare and appreciated, as well as being accessible.” Benefits Supervisor Sleeping was the first work from a series of paintings he painted in the 1990s when his work became more ambitious. The painting was sold by a private European collector and snapped up by an anonymous buyer.

This was the latest landmark in the contemporary art boom. According to Robin Duthy's Art Market Research, contemporary art has risen 700 per cent in value since 1985. The growth has become turbo-charged, with Hiscox, the insurer, estimating that contemporary art at auction increased in price by 55 per cent last year.

Fears of a downturn prompted by the gloomy world economic outlook have yet to be realised and the emergence of affluent buyers from Asia and Russia has inflamed demand.

The Freud sale on Tuesday was one of eight artist records broken that evening. Of 57 lots, 54 were sold for a total of $348.3 million.

Speaking before last night's sale, Francis Outred, of Sotheby's Europe, said that a fascination with British culture had helped Freud and Bacon to attain such high prices.

“However, the particular interest in Freud and Bacon is related to their pushing of the contemporary-art envelope. In comparison with a lot of contemporary artists, their work carries on the tradition of the great figurative painters whilst reflecting the philosophy of contemporary life. That's why they are so desirable right now.”

Best-sellers

The top-selling living artists:

1 Lucian Freud (born 1922) Benefits Supervisor Sleeping, sold for $33,641,000 (£17,251,795) yesterday

2 Jeff Koons (born 1955) Hanging heart (Magenta/Gold), sold for $23,561,000 in November 2007

3 Damien Hirst (born 1965) Lullaby Spring, sold for £9,652,000 in June 2007

4 Jasper Johns (born 1930) Figure 4, sold for $17,400,000 in May 2007

5 Gerhard Richter (born 1932) Kerze (Candle), sold for £7,972,500 in February 2008

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