Australia II today handed Liberty the worst defeat of an America's Cup defender in more than a century.

Liberty's crew immediately prepared for major changes to the weight and sail plan of the boat to try to make it competitive for the deciding race Saturday for international yachting's most prestigious prize.

On a brisk fall day on Rhode Island Sound the Aussies lost the start, but sailed past Liberty and took the lead halfway up the first leg.

Three hours and 22 miles later she finished 3 minutes 25 seconds ahead--the biggest victory margin for a challenger since 1871.

The victory tied the best-of-seven series at 3-3 and put Australia II in position to accomplish with one more triumph what hasn't been done by a foreign contender in the 132-year history of the Cup--win it.

In fact no other challenger has ever come close. Not since 1934 has a foreign boat won more than two races in the final series and none has won more than one since the competition switched to 12-meter yachts in 1958.

The Australians requested a lay day Friday to "relax," according to syndicate chief Alan Bond. The Americans won't rest. They will be trying to find an edge in speed, even if it means altering their boat in a bid to make it competitive with the white challenger with the radical, winged keel.

At a postrace press conference, veteran American 12-meter skipper Bill Ficker called Saturday's matchup "the most significant yacht race in history." There was no dispute.

Liberty skipper Dennis Conner was the victim of a dramatic wind change halfway up the first 4 1/2-mile leg of the six-leg course, just as he had been Wednesday when the Aussies dug themselves out of a 3-1 series deficit with a 1 minute 47 second victory.

Liberty was in the lead by several boat lengths under brilliant sunny skies, but allowed Australia II skipper John Bertrand to sail off about 150 yards to the left side of the course, alone. Then the wind veered slightly, picked up velocity where Australia II was and left Liberty in a hole. The challenger gathered speed, passed Liberty and two miles later had stretched the advantage to 2 1/2 minutes and 30 boat lengths.

"It just got better and better" for Australia, said Liberty sail trimmer John Marshall. "From then on it was pretty hopeless."

Conner's tactical maneuvering prompted immediate criticism among knowledgable sailors looking on from the spectator fleet. "He blew it," said one of the most outspoken, Peter Stalkus, who was navigator on Defender in her unsuccessful bid to be the U.S. selection for these races. "When you're ahead in a fluky northerly, you never, never want to split from the other boat."

But Conner said he felt he'd covered his foe well. "When we were ahead, every time Australia tacked (changed direction) we tacked on them, until they got ahead."

Conner said today's shifting conditions made it difficult to catch up because the leading boat parlayed the wind shifts to its advantage.

Of more concern to Liberty's crew than possible tactical errors is the apparent speed advantage Australia II has in almost all conditions. Today the winds were moderate at the start--12 knots--then fell off some and regathered strength from the southwest, the race finishing in 16-19 knots of breeze.

Liberty seems to go best against Australia in winds of 14 to 17 knots, said navigator Halsey Herreshoff. But even then, "We have to sail a perfect race to beat them," Marshall said.

Liberty's crew believes the Australians requested the lay day because weather predictions are for light breezes Saturday. Australia II, 4,500 pounds lighter and with more allowable sail area than Liberty, is all but unbeatable in light winds.

Marshall said if the predictions are reinforced Friday, Liberty will be reballasted. Lead would be taken from her keel and sail area added on the foresails and spinnakers in hopes of improving her light air performance, he said. Liberty was towed across Narragansett Bay to Cove Haven tonight, where the work would be done and where she would be remeasured and certified as a 12 meter by race committee measurers.

"That's a real desperation move," said Bruce Kirby, who designed Canada 1 for this Cup series. "Liberty is already much more tender (susceptible to gusts) than Australia II. If they lighten her they'll be gambling everything on winds under 11 knots."

Desperation may be all Liberty has left. In the seven races so far she has led Australia II around the first mark only once. In the history of 12-meter racing for the Cup since 1958, no challenger ever led the defender to the first mark before. This early lead is considered crucial, since the lead boat gets to dictate tactics.

Of Liberty's three victories, the first two profited from Australian gear breakdowns. Her only "clean" win came Tuesday, when Conner outfoxed Bertrand at the start, sailed a perfect race by every observer's account in winds ideally suited to Liberty, and won by a scant 43 seconds.

Today's apparently decisive tactical error by Conner on the first leg had Stalkus, the Defender navigator, wondering if the skipper is "so nervous about losing the Cup that he's overwhelmed." Stalkus said the wind shift from which Bertrand profited was obvious even several hundred yards from where the yachts raced.

Conner said anyone who saw the shift coming "must have X-ray eyes . . . because we didn't see it." And he said he will be delighted to be sailing Saturday in what he called "the yacht race of the century."