Fleming was forced to defend his tactics after a demoralising six-wicket Super Eights defeat by title rivals Sri Lanka here on Thursday.
He delayed taking powerplays fearing that, if the field were up, the 1996 champions would sprint to a win that would adversely affect New Zealand's net run-rate - the first tie breaker for semi-final qualification if teams are level on points.
Sri Lanka's win saw them draw level on eight points with reigning champions Australia as well as New Zealand.
Australia, who've played four games to their nearest rivals' five, top the table with a net run-rate of +1.51, Sri Lanka second with +1.35 and New Zealand narrowly behind on +1.27.
Fleming, whose side need at least one more win from their two remaining second phase games against fourth-placed South Africa on Saturday and Australia a week Friday, to be sure of a semi-final place, said: "The first 20 overs (of Sri Lanka's innings) we fumbled our way through when we needed a great start.
"From then on it was a fine balance between trying to put pressure on them and look after our run-rate."
Sri Lanka held New Zealand to 219 for seven, Chaminda Vaas removing the top order with three for 33 in an innings where Scott Styris's 111 not out accounted for more than half his side's runs.
In reply, Sri Lanka got off to a flyer and saw Sanath Jayasuriya (64) and Kumar Sangakkara (69 not out) put on exactly 100 for the second wicket.
"We were so far out of the game when they were one down," Fleming explained. "If we'd taken one more wicket we could have really squeezed.
"It's a fine line. You could lose 30 or 40 runs just like that in a tight game and that's going to affect the run-rate massively down the track.
"I wasn't willing to take the 10 overs of powerplay and lose the game in the 38th over when you could stretch it out for another eight overs," said Fleming after Sri Lanka won with 29 balls to spare.
Asked if he worried about net run-rate, Fleming replied: "You're dumb if you don't. As a captain you've got to look at all scenarios but by no means was there a question of giving the game up."
Defeat for New Zealand, against a Sri Lanka side missing injured fast bowler Lasith Malinga, saw them fail to equal their 2004 record of 10 straight one-day internationals wins.
"It was a poor performance," said Fleming who was lbw to Vaas for nought for the fourth successive time.
"The whole way through, particularly in our batting, they stunted any momentum we had. Scotty had a very good day, we just didn't support him.
"A contributing factor is that we haven't been put under the pump thus far.
"They fielded very well and they've got the most unorthodox attack, the toughest bowling attack in the world at the moment."
Looking ahead, Fleming, 34, said he didn't expect such a complicated challenge from South Africa.
"They are more orthodox with a pace attack, predominantly. But they are incredibly competent in what they do."