South Africa had crushed India by a huge margin of 135 runs in the first one-dayer at Durban on Wednesday.
But going by India's performance in the tour so far, South Africa cannot afford to relax as Mahendra Singh Dhoni and company have made a strong comeback in the three-match Test series, drawing it after losing the first Test by an innings.
"Just got to be very wary that this happened in the Test series. But I don't think it will. There seems to be a very good attitude amongst this one-day squad. They seem to have learnt their lesson. It's just important that they go in there and hit India hard from ball one again," Fletcher told media on the eve of the second ODI of the five-match series.
"And make sure they don't get complacent. India are quite an experienced side. Although they've got young players, if but they are pretty experienced. You got to be wary this fact. They know how to play out of these situations. South Africa have to be careful," added the former England coach.
Fletcher, however, is not happy with the home team's handling of the Power-plays.
"I've always believed they haven't really believed when they have taken it. If sides or captains want, there are a lot of possibilities on how to take them.
"It's just been a misinterpretation as how you play within the Power Play. Sometimes you've got to look at it as there are other advantages of taking it at a certain time," he said.
Talking about the changing face of the game in the wake of Twenty20 cricket, Fletcher said: "It (T20) had an influence right through cricket. The players are going to be more attacking, but 50 overs is a lot of cricket.
"If sides think they can go out there and adopt a Twenty20 attitude from ball one, sure on a given day they might be successful, but not over a period of time. They got to be very wary that in 50 overs there will be periods when you have to be more patient." Fletcher also said that international teams should have the ability to adjust to different conditions and playing surfaces and was satisfied with the Proteas team on this front.
"I believe South Africa matches everyone as far as one-day cricket is concerned. They haven't been fooled by the wicket they bowled on. They won't bowl on that wicket very often, but when they get on it they play very very well.
"When they go to India the wickets may be a bit slower and a bit flatter, but they just have to get used to it. They have to adapt a bit quicker. Bowlers at the international level have got to do that. They've got the skill and the ability to do that," he said.
Fletcher made no bones about the fact that South Africa was missing Jacques Kallis but said the upcoming all-rounders are really bright.
"Obviously they're going to have a problem with the all-rounder. South Africa always had plenty of all-rounders which has given them depth. They've got into the area that without Jacques Kallis they might need a bit of depth but the backup batters are good enough. All it needs a little bit more experience I guess," he said without taking any names.