Prada, one of the two top entries in the America's Cup challenger semifinals, suffered a stunning setback today when its mast snapped in half and came crashing to the deck on the first leg of a long-awaited race against regatta co-favorite AmericaOne.

The boats were battling up the course in moderate winds of 12 to 14 knots on a brilliant, sun-splashed day when Prada's towering, 110-foot carbon-fiber spar snapped without evident reason.

Skipper Francesco deAngelis said a steel fitting containing the shrouds that hold up the mast failed. "We broke the tip cup at the third spreader," he said, referring to the small fitting that failed and left the spar unsupported. It snapped like a twig midway up with carbon-fiber shards flying, and onboard microphones captured the verbal mayhem below as the crew took cover.

The break handed the high-flying Italians their second straight loss in the semifinals, in which six boats are racing over 10 days for two slots in the Louis Vuitton Cup challenger series. And it gave an easy victory to AmericaOne, Paul Cayard's top-rated U.S. entry, which only had to sail around the 18-mile course alone to collect a victory point while Prada headed to harbor under tow to replace the broken spar.

Prada had made it through three preliminary rounds of racing--29 races in all--with only three losses but now has lost twice in the first three days of semifinals. The team had sailed 2 1/2 years without breaking a mast.

The mishap cut short a brilliant, close race. Cayard barely avoided trouble of his own during prestart maneuvers when deAngelis forced him over the starting line prematurely with less than a minute to go.

Cayard, one of the world's best big-boat sailors, coolly steered a tight circle around the race committee's starting boat to get back to the proper side of the line, narrowly avoiding a collision with Prada as deAngelis sought to slam shut the tiny opening through which Cayard miraculously threaded his 80-foot racer.

Cayard hit the line with full speed as the gun sounded. deAngelis was 12 seconds behind, still recovering from the near miss and perhaps pondering how Cayard managed to escape without committing a foul. On-water umpires waved the all-clear, signifying no foul. Former Cup skipper Bruno Trouble watched in admiration and pronounced it "the best start of Cayard's career."

Things quickly got tight again as the two forged up the smooth waters of the Hauraki Gulf in a steady southwesterly. deAngelis played the shifting winds better and his boat showed better speed until two miles up the course, with the first turning mark in sight and AmericaOne a half-length ahead. Down came the mast.

The mast was the third to break in this 2 1/2-month-old regatta. deAngelis said Prada, with the biggest budget of any challenger at over $50 million and a fully stocked storeroom of spares, will have no trouble replacing the broken gear today and returning to the course Thursday for a scheduled matchup against French entry Le Defi Francais.

As the dismasting was unfolding, Dennis Conner's Stars & Stripes was carving out its third straight win and holding onto its position as lone undefeated boat in semifinals with an easy victory over hapless Le Defi Francais, which is now 0-3.

But Stars & Stripes was damaged in a collision with the French and may need a day off to repair a crack in the transom. French skipper Bertrand Pace was ruled at fault in the crash, which occurred at the second turning mark as the boats surged along at close quarters under straining spinnakers.

The penalty was Pace's second of the race and effectively ended any chance to upset the high-flying Stars & Stripes. He'd already committed an unpardonable sin during prestart maneuvers by entering the starting box a few seconds before the five-minute gun sounded to allow him to do so, incurring an automatic penalty.

The gaffe meant Pace was required to spin a 270-degree penalty circle at some time during the race. But his misfortunes weren't over. The diminutive skipper known as the Little General erred again six miles later when the judges ruled he illegally altered course as the two boats converged near the second turning buoy. They hit, and Stars & Stripes has a crack in the transom to show for it.

Stars & Stripes helmsman Ken Read had his crew put duct tape over the damage, then sailed to a 3 minute 6 second win while Pace did his penalty turns. Read asked for a day off to repair the crack properly when the boats returned to port.

The delay would give Read and his crew some time to reflect on their remarkable success so far. The low-budget, one-boat campaign has now beaten Prada, Nippon and Le Defi Francais and is in position to make a strong run for the two-boat challenger finals.

In today's third race, superior boat speed gave the Japanese entry Nippon an easy, 1 minute 31 second win over Dawn Riley's mixed male-female crew on America True.