The highlight of the Allan Stone collection at Sotheby's were 18 paintings by Wayne Thiebaud (b.1920) including Pies from 1961, sold for $3.5m (£2.27m).

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Even with an impressive 36 lots sold for over $1m each - including a $15m Jeff Koons and an $18m Warhol - Sotheby's mixed-owner evening sale of Contemporary art on May 10 was deemed a little lacklustre. Nine of the 58 lots offered were unsold, leaving the total of $111.2m (£72.2m), short of expectations of $120.8m-171.4m.

The last Pink Panther (one of Koons's most famous works from his Banality show of 1988) to appear for sale sold at Christie's in 1999 for a then-record $1.8m. This example of the porcelain sculpture combining a porn star nude with cartoon character was the artist's proof from a total edition of four, the others being in museum or foundation collections. It was being sold by publishing magnate Benedikt Taschen.

One of two lots in the sale to carry the third-party guarantee known as an 'irrevocable bid', it was estimated at $20m-30m but sold on a single nod for $15m (£9.74m).

Warhol's Sixteen Jackies - a group of panels from 1964 but assembled after the artist's death from the holdings of the Warhol Foundation - was again aggressively estimated at $20m-30m. It sold on the phone for $18m (£11.7m).

Sotheby's enjoyed more competitive bidding during their single-owner sale of Abstract Expressionism from the collection of the late New York art dealer Allan Stone (1932-2006). Christie's conducted the 64-lot Selections from the Alan Stone Collection sale in 2007, but this second slice of extraordinary holdings had crossed to their rivals at the eleventh hour.

The 42 lots (39 sold) offered by Sotheby's reached the top estimate of $47m (£30.5m). Nine works by Willem de Kooning, who Stone first bought in 1954 while still a student at Harvard, were relatively weak, but the 18 paintings by nonogenarian Wayne Thiebaud, who Stone represented from 1961, raced away.

Sotheby's $49m day sale (May 11) brought their week's business to $207.2m (£134.5m).

Christie's had guaranteed or found anonymous third-party backers for 13 lots in their evening sale, but a total of $265.2m (£172.2m) against expectations of $214.8m-299.2m suggests they had read the market correctly.

All but three of the 65 lots offered found buyers and two self-portraits by Andy Warhol, a previously unrecorded 1961 colour-infusion by Mark Rothko and Francis Bacon's Three studies for Self-Portrait of 1974 made up a quartet of $20m-plus lots of the type that had failed to materialise in the Impressionist and Modern art sales the previous week.

Across last week's sales there were more than 50 Warhols on offer. The most coveted proved to be Self-Portrait, a group of four blown-up and blue/lilac-stained mug shots of the artist wearing shades taken c.1963-64.

This prime-period work (estimate $20m-30m) came from the family of the late Detroit art collector Florence Barron, who had commissioned it from the artist at a cost of $1600. A long bidding battle between two telephone players finally ended at $34m (£22.1m). It is the most expensive lot to sell in any category so far this year.

As a whole, Christie's were well ahead of last year with their final aggregate - $319.6m (£207.5m) including the $54.4m Day sale on May 12 - their highest tally since May 2008.

Thirty-eight of the 50 lots offered found buyers at Phillips de Pury on May 12 for a total of $86.2m (£56m), just within the $84.5m-121.3m estimate but more than double the equivalent sale last year.

Eleven lots had received pre-sale guarantees, including Andy Warhol's turquoise Liz 5 (Early Colored Liz) from 1963, which was formerly part of the collection of Ileana Sonnabend (dispersed privately in 2008), but was entered for sale by hedge fund manager Steve Cohen.

Auctioneer Simon de Pury opened the bidding at $18m and took million-dollar-increment bids to $24m (£15.6m). Another of Warhol's 13 versions of the subject sold for $23.5m at Christie's New York in November 2007.

Meanwhile, Bonhams offered a key work by Isamu Noguchi - his wood and animal vertebrae Bone Sculpture from 1945 - as part of their New York sale on May 9. Estimated at $200,000-400,000, it set a house record for the Madison Avenue rooms at $880,000 (£570,000).

By Roland Arkell