The race became dramatic when rain started to lash the circuit in lap 45, 10 laps from the finish. Karthikeyan was running eighth at that time having picked his way through from 11th, passing the likes of reigning Super Formula champion Naoki Yamamoto around the outside of turn one after an intense battle.
Last year's winner Yuji Kunimoto was also dispatched at the same spot, this time with a late dive on the brakes on the inside. As the rain came down, some cars opted for wet tyres while others stayed out hoping that the rain would subside and the track would dry out - giving them an advantage over the wet tyre runners.
The call to stay out looked like paid off, especially after the Safety Car came out following a few spins and car crashes. But the rain intensified thereafter, and the safety car went in with four laps of the race remaining.
From then on, it was a struggle to hold track position with cars on wet tyres charging from behind. But despite a few moments, Karthikeyan managed to hold on and finish seventh, behind this year's 24 hour Le Mans winner Andre Lotterer.
"It was one of those races where any call that you make is more likely to be wrong than right - if we had switched to wets and the race was red-flagged or finished behind the safety car, we would have ended up outside the points. It's a shame that the podium got away, but we still managed to keep it together and bring home the points," he added.