Britain won its second race in the America's Cup final against New Zealand in tricky sea conditions on Wednesday, cutting the defending champions' lead to 4-2 and prompting skipper Ben Ainslie to say "the comeback is on" in the first-to-seven series.

The British outmanuevered the New Zealand crew led by Peter Burling in the final seconds of the pre-start in Wednesday's second race and shot across the line in their foiling AC75 to establish an early lead, building on their success in the first.

"We need to ride this momentum now and keep getting ourselves back in the game," Ainslie told reporters, adding that the British challengers would keep pushing for small performance gains in their high-tech boat.