Bendigo sailor Glenn Ashby has missed out on a second America's Cup title after his Team New Zealand crew was beaten by Oracle Team USA in Thursday morning's deciding race in San Francisco.

Team New Zealand looked set to claim the Cup when it opened up an 8-1 lead in the best of 17 series.

However, Oracle Team USA produced a stunning comeback, winning eight-straight races to clinch the title.

Oracle Team USA, skippered by Australian James Spithill, won Thursday morning's final race by 44 seconds.

Ashby, a 14-time world champion, won the America's Cup in 2010 with Team USA.

He was the trimmer for the Team NZ crew under skipper Dean Barker.

Barker, who watched his team fail to claim the one victory needed over seven days of racing, admitted the loss was hard to process.

"It is obviously very hard to fathom," Barker said.

"We didn't get the last one we needed to take it back to New Zealand. It is hard to swallow."

Spithill's victory has been hailed as a greater achievement than when John Bertrand steered Australia II from a 3-1 deficit to win the 1983 regatta 4-3.

In the longest competition in the 162-year history of the America's Cup, Spithill not only had to guide his crew through a do-or-die scenario for the past week, Team USA had to win two extra races after being penalised for illegal modifications made in the warm-up series.

"I don't know what went wrong," said New Zealand team chief executive Grant Dalton.

"We weren't quick enough in the end. My job is to support the guys because they are just really smashed ... well, the country is really devastated."

Spithill and fellow Australian crew members Tom Slingsby, Kyle Langford and Joe Newton hugged their teammates as their catamaran zipped past cheering, flag-waving fans on the shore of San Francisco Bay.

"Man oh man, what a race," Spithill said.

"Facing the barrel of the gun at 8-1 and what do these guys do? They didn't even flinch."

Adding to New Zealand's collective woe at the result - summed up in one-word tweet of "Bugger" by Prime Minister John Key - Oracle's chief executive is Kiwi sailor Russell Coutts.

Billionaire Larry Ellison, who bankrolls Spithill's victorious team, said Coutts had played a key role in ultimately foiling his compatriot's bid.

"He talked about sailing lower and faster instead of higher and slower," Ellison said.

"Russell Coutts has never lost an America's Cup - not a bad record.

"As long as Russell wants his job, we are glad to have him."