Italy's Luna Rossa Prada Pirelli, guided by Australian helm Jimmy Spithill, and Great Britain's INEOS Britannia have split the opening two races of the America's Cup challengers final.

The first yacht to seven wins will lift the Louis Vuitton Cup then try to topple defending champions, Team Emirates New Zealand, for the America's Cup next month.

"This is going to be a fight to the end," Spithill said after the score stood at one race each in the event staged off the coast of Barcelona.

Officials extended the opening two races from six to eight legs due to high winds whipping the foiling yachts along the Catalan beachfront at speeds of up to 50 knots (90 kph).

Both boats' helmsmen said the conditions were tough to keep the yachts on course.

"It was like racing a Ferrari on ice," Spithill said. "We were skating around out there."

The Italians' silver-hulled boat sailed a flawless first race which they led from the first gate, while Britannia touched down early to fall further behind. Luna Rossa crossed the finish line some 700 metres and 46 seconds ahead.

But the team led by British Olympic great Ben Ainslie pulled level with an impressive second race.

Britannia built a lead over the first leg and held off Luna Rossa to finish some 350 metres and 18 seconds ahead.

"This was a good one," Ainslie said. "It was obviously a better start. They kept coming at us, and kept it close."

Britannia took two wins over Luna Rossa in the double round-robin opening round this month.

The Brits then beat Switzerland's Alinghi Red Bull Racing while Luna Rossa eliminated American Magic in the challengers' semifinals.

This head-to-head is a rematch of the 2021 challengers final, when Luna Rossa beat Britannia 7-1 in Auckland before the Italian boat went on to lose to defender New Zealand 7-3.

Neither Britain nor Italy have ever won the America's Cup since its start in 1851.