Karthikeyan, starting from second on the grid, lost three places on the opening lap before recovering to finish third in the 43-lap race at the famous Suzuka Circuit.
Three-time Le Mans winner Andre Lotterer won the race, 9.4 seconds ahead of 2014 champion Kazuki Nakajima.
It was a welcome result for the 38-year-old Indian, who endured a tough opening season last year when he finished 13th in the Championship with five points.
"It (the result) is definitely pleasing considering the quality of the field. You are competing against guys like Lotterer, who won the WEC (World Endurance Championship) opening race just last week. Then you have former F1 drivers like Nakajima and Kobayashi (Kamui), who has been on the F1 podium. So surely a good result and promising start," Karthikeyan told PTI.
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The ex-F1 racer is driving Honda-powered cars this season and he feels it is a much improved engine.
"Unlike last season, Honda is competing with Toyota powered cars and that is good news for us. The car felt fantastic today. We have a strong package and now we need to make the most out of it."
"We can surely win with this car and that is the aim going into the next round. Plus I know all the tracks now unlike last season. So we will have to capitalise on all these factors and do the best we can," he said.
Karthikeyan's 2014 campaign was further dented by a 250 kmph crash during qualifying at Autopolis. It took months for him to recover from the accident but he is now fully fit and in good space.
"I am happy to be racing alongside some high profile driver. People tend to talk about the age factor but I have always believed in my ability. I will continue racing as long as I remain quick," he asserted.