Forced to sail mostly on guesswork after a technical glitch, Britain’s INEOS Team UK broke the magical 50-knot barrier in beating Italy’s Luna Rossa by 33 seconds on Saturday in the final round-robin race of the America's Cup challenger series at Auckland, New Zealand.

In doing so, Team UK, which had been a lame duck due to equipment issues in pre-Christmas racing, retained its unbeaten record after five races in the Prada Cup challengers series and to qualify automatically for the final.

On Saturday the British yacht Britannia came from behind to lead by one second at the final mark and held on to win a race which contained nine lead changes and was the best of the regatta so far, gripping to the last gybe.

When the boats came together on opposing gybes near the bottom of the last leg — Team UK on port and Luna Rossa with the right of way on starboard — Britannia slid just across the bow of Luna Rossa; only meters separated the yachts. The Italian team immediately protested but the umpires dismissed the protest and Britannia went on to a narrow win.

Team UK now moves directly to the Prada Cup challengers final while Luna Rossa will race the United States team American Magic in the semifinal. American Magic is currently making repairs to its race yacht Patriot which was severely damaged in a capsize a week ago.

The British team had a major problem in the lead-up to Saturday's race with the Cunningham — the downhaul system which controls the shape of the sail. Its setting changes upwind and downwind and as the wind shifts but after a malfunction before Saturday's race, Team UK was forced to guess the right settings and rely on instinct during the race.

As they rushed to make repairs they were able to take advantage of a new rule, just brought into force, which allows both teams one chance to call for a 15-minute postponement to the start.

The race start had already been delayed on several occasions as the wind shifted from southwest to west, forcing the race committee to adjust the orientation of the course.

Team UK skipper Ben Ainslie applauded his team's response to the challenge foisted on it by the Cunningham mishap.

“Our mainsail trimmer did an awesome job to try to keep us powered up downwind,” Ainslie said. “We were losing a little bit as you probably saw, had a high fast mode. We guessed the setting just about right for the upwinds and it was nice to have enough pace to get around.

“We were missing one of the key settings for the power of the boat so the guys just did an awesome job getting us around the track in one piece and getting the win.”

Britannia and Luna Rossa were side by side across the start line with the British yacht to windward. Ainslie pushed the Italian yacht to the left-hand boundary before tacking away.

When the boats first crossed, Luna Rossa was just in front of Team UK but the British boat was slightly faster. It approached the top mark just inside Luna Rossa and rounded two seconds in front.

Luna Rossa showed its speed advantage downwind and reached the second mark nine seconds ahead. It extended on the second upwind leg and led by 19 seconds at the top mark.

Team UK made some gains on the fourth leg of the leg of the six-leg race, cutting Luna Rossa’s lead to 10 seconds. It continued that improvement on the penultimate leg, finding speed on the left of the course as the wind continued to shift.

The British yacht turned the final mark just one second ahead of Luna Rossa, choosing the right marker while the Italians chose the left. That allowed Luna Rossa to spend longer on the favored gybe but the British boat held its advantage. It gybed back to the left and then, almost laying mark came back on port and just squeezed across Luna Rossa’s bow.

“It looked good initially out of the gybe and it looked good most of the way across,” Ainslee said. “Then we both went for a soak mode so I guess their soak mode was a little better than ours with the Cunningham off. So it was about as close as you want to get it.”

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