The America's Cup is set to come down to a sudden death final race after Oracle Team USA came from behind to win Wednesday's second race and square the series at 8-8.

However, forecast strong winds threaten to postpone the decider, scheduled for Thursday.

On Wednesday Emirates Team New Zealand won the start and led by a close five seconds at the first mark on San Francisco Bay, then extended slightly to lead by seven seconds by the second mark.

However, Oracle overtook the New Zealanders on the first cross on the upwind leg to seize the lead, streaking away to a big 57 second advantage by the third mark.

Oracle eventually won by 54 seconds, for skipper Jimmy Spithill's seventh straight victory over Dean Barker.

An aggressive start cost Team New Zealand dearly in the day's first race, with Oracle capitalising on twin penalties to win by 27 seconds.

Barker and Aotearoa were hit with a crippling slowdown penalty for not giving way at the start, then another for touching Oracle shortly afterwards.

The holders raced out to a comprehensive lead, which they never relinquished.

Barker is taking responsibility for the first race failure, but says the New Zealanders were powerless to stop the Americans in the second.

"It was the first time that I think we've recognised that there was a condition where we're not as strong as we need to be," he says.

Oracle skipper Jimmy Spithill says his team has come back from a very deep hole and will come out ready to fight on Wednesday.

Spithill says his crew is riding a wave of momentum that will continue into the deciding race.

Auckland Yacht Club Commodore Steve Burrett has slated the America's Cup rules, saying they do not adhere to the principles of yachting.

He is upset a rule governing television coverage means Saturday's win over the American team was discounted because the race took too long.

Mr Burrett says the American team have used their bottomless financial pit to copy aspects of the New Zealand boat and improve their performance in eight days.

Warning from psychologist

Massey University professor of sports psychology Gary Hermansson says it is characteristic of New Zealanders to have their sense of self-esteem tied up in sport and it becomes a major issue when their team is not doing well.

He says people need to keep their emotions in check, put the regatta in perspective and remember it is only a boat race.

Professor Hermansson says New Zealand became something of a sad case across the globe after the World Cup Rugby loss in Cardiff in 2007, with people in other nations puzzled about the period of national mourning that followed.

TAB bets popular

The TAB says more than 85% of bets since Wednesday's races have been on Oracle to win.

The agency had at one stage closed betting because it looked certain Team New Zealand would win.

TAB America's Cup bookmaker Kieran McAnulty says because Oracle was at one point paying $12 to win, it will become a large liability for the agency.

Mr McAnulty says Oracle is now paying $1.25 against Team New Zealand's $3.50.