British sailing legend Ben Ainslie hailed an "epic turnaround" on Friday after his INEOS Team UK charged to a 2-0 lead on the opening day of the America's Cup challenger series in Auckland.

Critics labelled the boat "the lamest of lame ducks" when it lost all of its six warm-up races last month, failing to finish three of them.

But four-time Olympic champion Ainslie bounced back from the embarrassing flop to claim victory over rival challengers American Magic and Italy's Luna Rossa on day one of the Prada Cup challenger series.

The series will determine which of the gravity-defying 23-metre (75-foot) yachts - which fly above the water balanced on hi-tech foil arms - will race defending champion Team New Zealand for the America's Cup in March.

The Prada Cup has 17 race days, meaning Team UK remains a long way from claiming the challenger's spot.

However, the opening day success shows detractors were wrong to write off Ainslie's challenge.

"It feels a little bit better than the six losses or whatever it was," he said after winning the first leg.

"It nice to find the pace when it counts."

In the opening leg against American Magic, Team UK went through the first gate 11 seconds ahead and steadily built the lead to win by one minute 20 seconds.

Ainslie's boat again led from start to finish in the second leg against Luna Rossa, when the Italians struggled to stay up on their foils in light winds to lose by 28 seconds.

Not only was the British boat faster, it also manoeuvred better than in the warm-ups, with Ainslie praising a "nice team effort".

He said technical staff onshore had worked around-the-clock since Christmas to rebuild much of the boat and make it competitive.

"Back at the dock our designers, our engineers, our shore team, our boatbuilders, they've had an epic two or three weeks turning this boat around," he said.

"We're a long way from out of the woods, that's one good race (but) it's a huge motivator for our team. Hopefully we can build on that momentum now and keep moving forward."

Ainslie helped engineer an astonishing comeback in the 2013 America's Cup, when he joined Oracle Team USA as a tactician mid-race and they stormed back from 8-1 down to beat Team New Zealand 9-8.

The Prada Cup runs until 22 February, beginning with a series of round-robins.

The winner proceeds to the final, while the other two challengers face off in a seven-race semi-final.

The final begins on February 13 and is a best-of-13 contest, with the first yacht to win seven races victorious.

The winner then faces Team NZ from 6-21 March to compete for the main prize.

Source: News24