New Zealand 241-4 beat England 237-7 by six wickets
Fleming was in awesome touch
New Zealand went forward to the final of the NatWest Series on 10 July after hammering England by six wickets.
Andrew Flintoff hit his first one-day century for England but they had to bat first in tough, damp conditions early on and made only 237-7.
And an excellent innings of 99 by Stephen Fleming saw his side canter to victory in the 48th over.
England must beat West Indies on Tuesday by a big enough margin to claim a bonus point to reach the final.
After early morning rain, the toss looked a particularly important one to win and it was Fleming who called correctly.
He had no hesitation in asking England to bat first under cloud cover and on a pitch that had a greenish tinge and plenty of grass on it.
England opted for a watchful approach but were unhinged when they lost their best batsman at this form of cricket, Marcus Trescothick, to a needless run-out in the fourth over.
The left-hander was dropped by wicket-keeper Gareth Hopkins who fumbled the ball down to third man.
There was an easy single, but Trescothick unwisely attempted a second and was beaten by Ian Butler's return from the deep.
Michael Vaughan and Robert Key then gave their wickets away after playing themselves in - Vaughan, on 12, driving Jacob Oram to cover while Key (18) edged Scott Styris to Hopkins.
From 57-3, Strauss and Flintoff initially had to build slowly and even at the 30-over mark the run rate was still modest with the total 105-3.
But as batting became easier, so the big strokes came out - Strauss driving on both sides of the wickets to good effect - while Flintoff favoured the leg-side and added some booming shots down the ground.
Andrew Flintoff thrilled the crowd but his runs were all in vain
When Strauss finally fell to Butler, Paul Collingwood produced an important 20 off as many balls and kept Flintoff company as he reached his century.
Oram finished with superb figures of 2-27 from his 10 overs while Butler took two wickets in the final over to end with 3-57.
The sun was out at the start of New Zealand's innings as Fleming and Nathan Astle had things largely their own way early on.
Sajid Mahmood had a nervous debut and the two batsmen were quick to latch onto anything erring in line or length from him as the 22-year-old conceded 56 in his seven overs.
England did not always have luck going their way and had to wait until the 27th over before claiming their first wicket - Harmison yorking Astle and bowling him for 53.
Hamish Marshall, who had dropped Flintoff before he had scored and Strauss on one, found better form as a batsman.
He had hit 55 off just 51 balls before being one of three men to lose their wicket with the final hurdle in sight.
England: M Vaughan (capt), M Trescothick, R Key, A Strauss, A Flintoff, P Collingwood, G Jones (wkt), D Gough, SJ Harmison, JM Anderson, S Mahmood.
New Zealand: S P Fleming (Capt), N J Astle, H J H Marshall,
S B Styris, C D McMillan, J D P Oram, C Z Harris, G J Hopkins (Wkt),
D L Vettori, J E C Franklin, I G Butler.