The hosts cruised to a nine-wicket victory over the Proteas in the first game of the three-match series at the Essex County Ground last night.
Having lost the toss, South Africa managed just 89 for four off 20 overs before unbeaten half-centurion Edwards made a mockery of that total, guiding her side to victory with 39 balls unused.
But Edwards believes the tourists, who also tasted defeat when the two teams met in the Women's ICC World Twenty20 in Bangladesh earlier this year, will perform better in Northampton.
She said: “We want to play our best cricket. If we bowl a team out for 90 and we get it, that is a clinical performance and that is all we can control.
“I'm sure the South Africans are going to come back hard at us at Northampton. They are a good young team who I am sure are going to improve.”
Edwards was the star of the show as she continued her fine limited-overs form this summer.
The 34-year-old struck a fifty and an unbeaten century, her ninth in 50-over format, during the 2-0 Royal London One-Day Series success over India.
And last night she led from the front again by blasting 62 off 54 balls to move England 1-0 ahead in the rubber.
Edwards said: “I am hitting the ball well. It is important when you are in this kind of form that you continue to do that and not give your wicket away.
“I keep telling myself keep batting and I guess I will bear the fruits, which I have done.
“Hopefully I can carry this form into the rest of the games come Wednesday and Sunday.”
Seamer Katherine Brunt looked back to her best at Chelmsford in only her second appearance for the national team since a lengthy back problem.
The Yorkshirewoman, opening the bowling with Anya Shrubsole, returned figures of 1-16 from four tidy overs.
Edwards has warned not to expect too much so early on in her comeback, though, adding: “She is chomping at the bit to get out there and get back to her best.
“I guess it is just us reigning her in now and not expecting too much too soon.
“She has come back from a serious back operation and it is nice to see her out there enjoying the crowd and putting in a great performance.”
