A New York State Supreme Court judge delivered a Thanksgiving eve jolt to America's Cup holder Dennis Conner yesterday when she upheld New Zealand's demand for a Cup showdown in huge, 120-foot sailboats next summer.

Judge Carmen Ciparick ruled that Conner's Sail America Foundation must "accept the challenge, forfeit the Cup or negotiate agreeable terms with the challenger."

The ruling sets the stage for a head-to-head grudge match off San Diego next August between the two archrivals in boats of a scale not seen since the turn of the century.

Sail America conceded in a prepared statement last night that the ruling "paves the way for an America's Cup regatta to be raced . . . within the next 10 months."

Sail America had vowed to appeal the ruling if it lost, but last night chief executive officer Malin Burnham said only that the foundation will "evaluate the ruling and stand true to our obligations." He said he was "disappointed by the decision."

Michael Fay, the banker from Auckland who backed New Zealand's strong Cup challenge last winter in Fremantle, Australia, issued the new challenge July 15 after carefully reading the 100-year-old Cup Deed of Gift with his lawyer.

"It must have been the first time anyone read it in years," Fay said. They found that the right to call a race for yachting's top prize, long dictated by the Cupholder, was in fact guaranteed by the Deed to any challenger at any time, as long as he brought an acceptable boat no longer than 90 feet on the waterline and gave 10 months notice.

This simple, old-fashioned challenge formula had been abandoned during the New York Yacht Club's 132-year hold on the Cup, which ended in 1983. NYYC set the terms of racing, which since the 1950s has been every three to four years in 12-meter yachts half the size Fay has in mind.

Conner, who won the Cup in February, planned to follow the NYYC formula and had scheduled 12-meter racing in 1991 off San Diego.

But Fay short-circuited the plan, formally challenging under terms of the original deed and demanding a race in 10 months. When Sail America failed to respond, he took the case to New York Supreme Court, which has administered the Deed of Gift since the late 1800s.

Ciparick ruled that Fay's literal reading of the Deed was correct and Conner had to honor his challenge. "For the court to decide otherwise," she wrote, "would be to allow the holder of the America's Cup to virtually unilaterally dictate conditions for future competition."

That, she wrote, "would clearly violate the donor's intent."

Unless the ruling is overturned on appeal, Conner must build a 90-foot-waterline boat fast. Fay's entry, designed in Annapolis by transplanted New Zealander Bruce Farr, is already being built. Australian millionaire Alan Bond also is building a big boat, in hopes he can compete next summer, and the British and Japanese are considering jumping in.

Said San Diego Yacht Club Commodore Fred Frye last night, "The yacht club is firm in its resolve to defend the Cup successfully whenever it is necessary." Sail America officials said a committee would decide today on a formal response.

Fay said by phone from Auckland, "I've read the decision. It's very considered and very complete. It upholds the best traditions of the Cup and vindicates what we said.

"New Zealand is very pleased with the judgment. We've won one round. Now we must get on the water and win a match for the Cup in 1988."

Fay said his boat will be launched March 27 and shipped to San Diego in May for summer-long trials.

Conner was the first American ever to lose the Cup when his Liberty fell to winged-keel Australia II in 1983, but his Stars & Stripes regained the Cup last February in Australia. New Zealand, in its first Cup try, had the best record in the competition until squaring off against Conner in the challenger finals and losing, 4-1.