According to CNN, the 81-year-old Hockney is considered "one of the world's most popular artists".
Another of Hockney's works, a 1990 landscape "Pacific Coast Highway and Santa Monica", fetched $US28.5 million at Sotheby's in May. Starting the piece in 1971, the pair had already started going their separate ways.
Morgan Long, senior director of art investment house Fine Art Group, hailed "a great result for Christie s", saying it achieved its predicted $80 million price "through a combination of clever marketing and what looked like sheer determination on the part of (a) phone client to take the painting home".
"We weren't the only ones going after this painting", he said. After it was announced last week, "everyone was ringing to leave a bid", said Christie's President Jussi Pylkkanen, the evening's auctioneer. The final price of $90.3 million includes a $10.3 million buyer's premium.
The 1972 painting entitled Portrait of an Artist
The painting was bought by a client of Marc Porter, chairman of Christie's Americas. "I always loved swimming pools, all the..."
The first man depicts Hockney's former lover and muse, Peter Schlesinger, who was one of his students at the University of California, Los Angeles. The idea for the painting struck Hockney after two other paintings in his studio in Powis Terrace, Notting Hill, became juxtaposed in his mind.
Christie's had estimated the painting pre-sale at United States dollars 80 million and insisted it was "one of the great masterpieces of the modern era".
This week's sales, which kicked off November 11 and included Impressionist, modern, postwar and contemporary art, have generated $1.96 billion, eclipsing the $1.8 billion estimate.
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