"Their confidence must be low, so we must hit them hard early on," Fletcher told reporters.
South Africa have endured a miserable run over the past 18 months and they have slumped from second to sixth in the ICC Test ratings.
Having been beaten 1-0 earlier this month in the two-Test series in India, South Africa need to regroup quickly if they are to avert a similar fate against England.
"They are a proud cricketing nation and they can hit back hard," Fletcher said.
"We need to make sure we just concentrate on looking after our own jobs."
The England squad leave for Potchefstroom, 140 kilometres south-west of Johannesburg, on Friday for a three-day game against a South African 'A' team starting on Saturday.
With the first Test beginning on December 17, Fletcher said it was important for the England team to build up momentum.
"Steve Harmison looked pretty effective in our first warm-up game against the Nicky Oppenheimer XI, and the other bowlers probably need just two or three more sessions and then they'll be ready for the Test," said Fletcher.
