Slingsby who had won five rounds in a row before being beaten by the Kiwis in Plymouth, says it was only a matter of time before the they clicked.
"We knew they were lurking and were improving event-to-event," Slingsby said.
"Their level jumped up a long way and we knew the Kiwis were coming to the top of the fleet soon and in practice, and it was just a matter of time they got that off their back that they hadn't made a final. They are going to be a contender this year for sure."
Spithill, whose US team is languishing in seventh place, has been impressed with the drastic improvement shown from the Kiwis.
"Oh, look, they're some of the best guys in the world." Spithill told the Herald.
"I've had some great racing against Pete, Blair and the rest of the squad there. So, in some ways, we were a little surprised it took them until this point, but now you can see it clearly, they're sailing very well and now they are a force to be reckoned with."
Earlier this week, Spithill was criticised by SailGP chief executive Sir Russell Coutts for the poor showing by the Americans this season. Having joined the team after the first season in 2019 when the Americans finished last, Spithill took the team to third overall last year with seven race wins, making six finals. But Spithill is yet to make the podium race in the first three regattas this campaign and Coutts suggested there would have to be changes in the American crew if the fortunes didn't improve.
Spithill laughed off Coutts's criticism but admitted there was no easy fix.
"That's great, man. We love a bit of banter and a little bit of pressure, and look, in a lot of ways I actually think we batted above our average in season two," Spithill said.
"In season one, the team was in last place. I got involved in season two with a completely brand-new team and we went to the final. A lot of other teams didn't, the Kiwi team, the British team, there's some big names there.
"I think for the US, I mean, clearly, if you look at the high performance talent around the world, you'd say Australia and New Zealand, in terms of high performance, youth sailing, the UK, having spent obviously time in Italy, they've made a big investment.
"I think the US has been on the backfoot there. But now they've recognised that. So, they need to supplement their pro college sailing where there is no foiling and make an investment. Certainly, having just lived in New Zealand, that's something you guys have done very, very well and that you have a lot of talent there coming through that's been exposed to high performance sailing and racing."
It was a mixed bag for the New Zealanders in practice racing overnight. A hydraulic issue kept them out of the first race before they sailed flawlessly to win the second and finished fifth in the third.
Light winds are forecast for the opening day of racing tomorrow, with the first three fleet races scheduled between 1.30am and 3am NZ time. The format sees two more fleet races early Sunday morning (NZ time) before the podium race featuring the top three teams.
There are a further seven rounds in season three in St Tropez (September 10-11), Cadiz (September 24-25), Dubai (November 12-13), Singapore (January 14-15), Singapore (February 18-19), Christchurch (March 18-19) and San Francisco (May 6-7).
Tickets for the New Zealand leg are available from October 4.
