Australian skipper Tom Slingsby has endured a frustrating day in America's Cup challenger action in Spain.

Italy took a 2-0 lead over the Slingsby-led American boat, while the British crushed the Swiss as the semi-finals started on Saturday.

Luna Rossa Prada Pirelli's pair of close wins over NYYC American Magic, the first by seven seconds and the second by 18, put them in a commanding position in their first-to-five-wins series.

Luna Rossa swept American Magic 4-0 at this same stage in the 2021 America's Cup in Auckland. The Italians then dispatched the British in the challenger final.

American Magic was without helmsman Paul Goodison, who broke five ribs when he fell on the boat last week. Lucas Calabrese was skippering in his place across from Tom Slingsby.

Slingsby said despite the hardship his team has been through losing Goodison, not all hope was lost considering the fine margins in their two races with the Italians.

"I'm not happy that we've had two losses. That stings," Slingsby said.

"But at the same time, I think of where we were a week ago, (when) we were sort of probably down a bit on speed against Luna Rossa … and we lost our helm Paul Goodison. It kind of felt like everything was on the stack up against us.

"Today, we made a couple of mistakes and let them get through, but it was a very open race. We had the boat speed, we had the manoeuvres, (and) Lucas did an amazing job. So that's exciting moving forward."

INEOS Britannia brushed aside Switzerland's Alinghi Red Bull Racing to also go up 2-0 on the race course off the Barcelona beachfront. They won their first race by over two minutes and the second by 1:37.

The British, as the highest-scoring team in the opening round-robin, picked the Swiss for their semi-final. Britannia finished the round-robin with an 8-2 record, while Alinghi barely avoided elimination at 3-7.

That difference in performance held true on Saturday, when the Swiss had a better start only for the Brits to stream ahead on the second leg and never look back.

"It's good to get a couple of wins on the board," Britannia helmsman Dylan Fletcher said.

"Like all teams we have our weaknesses, but we will keep them close to our chest."

The last boat standing will challenge defending champion Team Emirates New Zealand in the America's Cup final in October.