Four-time America's Cup winner Brad Butterworth has warned that Oracle could still quickly become competitive when the battle for the Auld Mug resumes.

Butterworth, in Bermuda and alongside Peter Montgomery in driving Radio Sport's live race commentary, cautioned against too much Kiwi optimism among fans.

"It looks pretty convincing," Butterworth acknowledged in talking to Mike Hosking on Newstalk ZB.

"(But) I don't want to jinx this thing. Things can change and they've got a week that they can change the way that boat is configured.

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"The boats are all the same weight. A lot of things are the same. There are just little things that Team New Zealand are doing better.

"Those guys (Oracle) have got plenty of resource and plenty of power, and they can change things if they have to."

Listen: Brad Butterworth on the Mike Hosking Breakfast

The five days Oracle will have to analyse how the Americans replicate their feats of San Francisco four years ago will include sailing as well as technical refinements.

"I think the Team New Zealand, (Peter) Burling and Glenn Ashby, they're sailing their boat differently to the other guys," Butterworth said.

"Obviously they are sailing it better. But because everything is televised you can look right into the boats, how they are sailing, they'll be trying to learn something to help them lift their game."

Then there is the Jimmy Spithill factor, added Butterworth.

"He's an Aussie battler, that guy. Nothing is too hard for him," said Butterworth who was tactician onboard two Team NZ and two Alinghi America's Cup wins.

"But Burling has done a great job.....hopefully four more races."