American Magic were sailing towards their first win of the America's Cup Challenger Selection Series when disaster struck on the Hauraki Gulf.
The boated helmed by New Zealander Dean Barker capsized just after they rounded the final mark in their race against Italian entry Luna Rossa on Sunday.
Patriot was on foils and flying when the crew lost control and the AC75 went over.
The boat could not be righted quickly. All 11 crew appeared to be accounted for as chase boats moved in to assist.
American Magic have previously capsized 19 times in their trial boat (the boat before their first generation AC75).
American Magic held a narrow six second lead at the end of the first leg, extending that to 42 seconds after four legs.
At the top gate for the last time, the breeze had gone so far left that the left hand mark was the only choice.
The America's Cup website reported that resulted in a very difficult manoeuvre, especially with a squall sweeping across the course.
Patriot made it through the tack, but as they came out the leeward running backstay was not released, preventing the mainsail from being eased.
With sail pinned in and the gust now on them, American Magic's bow reared up and a capsize was inevitable.
The Americans were forced out of the race that they were leading, giving Luna Rossa their first win of the day.
American Magic had been struggling in this regatta and had lost three previous races - to leave them at the bottom of the standings.
Racing took place on the northernmost race course, course A, for the first time in the regatta as winds reached nearer the top end of the wind range which spans from 6.5 knots to 21 knots.
The boat was towed back to its base for repairs.