India bowled their hosts out for 116, a record low score for South Africa on home soil, and then easily chased down the runs with exactly 200 balls to spare.
The tourists won the final Twenty20 International on the same wicket on Thursday, and Rahul says he believed it would be a game for his spinners.
Instead seamers Arshdeep Singh (5-37) and Avesh Khan (4-27) both recorded career-best figures.
"It was completely different to what we expected," Rahul said at the post-match presentation. "We had planned to bring the spinners into the game early after what we saw in the T20 match.
"But there was a lot of help in the wicket and the boys did really well to put the ball into the right areas.
"It is my first ODI win here (South Africa) as a captain. When I was here last time (January 2022) we lost all three, so it is good to get that victory under your belt."
Both sides have rested players ahead of a two-match Test series starting on Dec. 26. There were several new faces in the Indian team, including debutant opening batter Sai Sudharsan, who took his chance with an unbeaten 55 from 43 balls.
"There is a lot of cricket being played these days. You have to prioritise formats and at the moment it is Test cricket and T20s a little bit," Rahul said.
"That is the modern-day game, but we believe whoever is here will perform for their country."
South Africa never got going in the game with bat or ball and have much to think about ahead of the second match in the three-game series in Gqeberha on Tuesday.
"Credit to their bowling attack with the lateral movement," home captain Aiden Markram said. "We weren't able to settle and build partnerships. It was a problem right from the start and we couldn't pull things back.
"We expected it to go around (seam) a little bit. Generally in the day games it does do something for five to seven overs. But today it went on for longer and we couldn't get in."
