England lost their number one world Test ranking after losing 2-0 to the Proteas but Cook says that he saw enough in the final Test to remain confident in the ODI side.
"We played some really good cricket at Lord's even though we didn't get the result," Cook told the Evening Standard.
"In any one-day series it's important to start well. We saw against Australia we got our noses ahead and they found it very hard to come back. When you can do that it makes it a lot easier going into the later games."
England play five ODIs and three Twenty20 games to complete the South Africa series.