"I was looking at options. One was to go to Indycar in America. I heard there were new cars coming to Japan and decided to go and test them in December. I was fastest in testing in December in Fuji and then the team invited me again in January and it went really well," said the ex-Formula One driver, who is being supported by Tata Group.
"The cars are really fast so I decided on this championship. There are several (former) F1 drivers driving here so it's a very tough competition. It will be quite a challenge," he told reporters here.
Karthikeyan will drive for Team Impul Hoshino Racing and said with new cars this year, the format should be on par with the glamorous Formula One in terms of speed and times clocked.
"(I was) Pleasantly surprised that we have a new car this year, the Dallara SF 14, 2.0 litre engine. Compared to Formula One last year, we were three seconds off in similar tyres.
"It is going to be really fast with the format being the same as F1 with a pit stop and a luxury of a fuel stop as well. Quite a lot of strategies will come in to place."
The 37-year old said no racing event can be compared to F1 but qualified the statement by saying he didn't find it exciting to be in F1 as the cars are not on par with each other.