Ben Ainslie Racing battle the rough seas in day two of racing in the America's Cup challenger series semifinals.

Team NZ sailors and debris in the water after the boat capsized in the Great Sound.

Crew boats respond after Team New Zealand capsize in the start box of their America's Cup challengers series semifinal race against Ben Ainslie Racing.

Team NZ sailors hold on as their craft is righted after a capsize during the America's Cup challenger series semifinals.

Team New Zealand sailors hang on after the boat capsized in the Great Sound.

Team NZ sailors are suspended in mid-air with the boat on its side on the Great Sound in Bermuda.

A Team New Zealand work to right their vessel after they capsized before the start of their race against BAR.

A Team New Zealand crewman in the water after their disastrous pitch-pole.

Team New Zealand's main sail is in tatters after their capsize.

Team New Zealand sailors are trapped in their pods after the their boat pitch-poled.

Team New Zealand lies stricken in the water after capsizing.

Disaster as Team NZ pitch-pole in the Great Sound off Bermuda.

Team New Zealand have lost a race to their British rivals in dramatic circumstances after capsizing in the start box amid wild conditions for the America's Cup challenger series semifinals.

Trailing their British opponents to the line in their second race of the day, the Kiwi catamaran flipped spectacularly forwards after its hulls lifted up and then plunged sharply into the water on Tuesday (Wednesday NZ time).

In sailing terms, the incident is called a pitch-pole. Some of the Team New Zealand crew were flung into Bermuda's Great Sound, while others were left clinging on in their race positions, almost hanging upside down.

GILLES MARTIN-RAGET/ ACEA 2017 Team New Zealand flip spectacularly in the start box.

Despite Team NZ failing to get across the start line, the race would count with the win awarded to Sir Ben Ainslie's British syndicate. Team NZ, having won the first race earlier in the day, hold a 3-1 lead in the best-of-nine series.

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Support boats of both teams were quickly on the scene and all six crew members appeared to escape unharmed from the frightening incident.

"All sailors accounted for and safe after @EmiratesTeamNZ capsize. Boat is making its way back to dock to assess damage," Team New Zealand posted on Twitter.

All sailors accounted for and safe after @EmiratesTeamNZ capsize. Boat is making its way back to dock to assess damage. — EmiratesTeamNZ (@EmiratesTeamNZ) June 6, 2017

A stunned onlooker like everyone else, Ainslie said helping their opponents in the case of a capsize had been something they had discussed as a team prior to the start of racing, given the strong winds and choppy seas they were faced with.

"I didn't actually see the capsize itself, we'd just come off the start-line," the multiple Olympic champion said in the aftermath.

"Thank God, it sounds like everyone's OK ... that's the most important thing in these situations.

GILLES MARTIN-RAGET/ ACEA 2017 The Kiwi support crew arrives to help after the dramatic moment.

"What an incredible day of racing. In 30 years of racing sailing boats, I've never experienced anything quite like that. To be honest, this is the first time we've sailed this boat in this much wind, so we're learning as we go. It's just absolutely full-on, it's mindblowing."

While the Kiwi crew got away largely unscathed, the news was not so good for their high-tech catamaran.

Most of the clearly evident damage was thought to be cosmetic, but their did look to be a breakage towards the top of what was the team's replacement wing sail.

It meant Team New Zealand had two broken wing sails, having suffered damage within their main wing sail just before racing was due to start for the day.

The Kiwi outfit worked superbly under huge time pressure to fit the replacement in barely enough time to make it out onto the course for their first race.

They then produced an equally impressive on-water performance to come-from-behind and claim victory in highly challenging conditions, handing them a 3-0 lead in the first-to-five semifinal series.

The British syndicate was awarded the win in race two after Team New Zealand's dramatic flip, closing the margin by one.

There was some potential good news for the Kiwis, with even stronger winds forecast for day three of the challenger semifinals.

That could mean a postponement of racing, which can't take place above the upper limit of 24 knots.

Winds were recorded at 21 knots and gusting close to that upper limit just prior to Team New Zealand's crash, which came at the end of a chaotic day which saw both boats in the other semifinal series also taking damage.

Team Japan won two-from-two against Artemis to push out to a 3-1 lead in the race to five wins.

But both boats had big moments in their first race, causing parts of their fairings to be ripped away from the inside section of their machines.