The fast bowler took 5-44 to leave India 208-8 when bad light and rain brought a premature close to day one at Centurion outside of Pretoria.
It was the first test for Rabada since South Africa beat the West Indies in Johannesburg in March but he showed no ill- effects from his long absence from first-class cricket.
"I was pretty focused in my training and I knew what I wanted to get out of my training but I think days like this happened in cricket, where sometimes you get the ball in the right areas and you’re successful and today was just my day," he told reporters.
"I'm pretty happy with the way I bowled. All week, the ball has been coming up nicely so I came into the match quite confident and I'm just glad that all the focus and hard work paid off. My mind and body was feeling good coming into this test series."
Rabada made the initial breakthrough against India and took three quick wickets after lunch to stop touring side wresting back control, including having Virat Kohli caught behind with a superb delivery.
"It just swung away late and most of the time he seems to just cover that channel," Rabada said. "He’s a fantastic player and when you play against him you really have to be on, so I’m glad I got the faint edge there."
KL Rahul, however, continued to provide resistance and was 70 not out at the close.
"India have a quality batting line-up with quite a bit of experience so 208-8… we would have taken that before the game started and especially because we sent them into bat. All in all, I didn’t think we bowled badly but I think there's room for improvement," Rabada added.
Check out the scorecard from the first test with Flashscore.