Unfazed by the pressure of an approaching triple hundred, Nair went about his business in the final session on Day 4, and calmly slotted a ball to the boundary when he was at 299 to complete his triple ton.
His unbeaten 303-run knock put him in the same bracket as Garry Sobers and Bob Simpson on account of converting a debut century into a triple ton.
Reflecting on his knock, which helped India take charge of the Chennai Test, Karun Nair recalled a near-death experience in Kerala earlier this year, when he survived a boat accident despite not knowing how to swim.
"I didn't know how to swim. People there rescued me and I was lucky to live again (about the boat accident in his home in Kerala)," Karun told the official broadcasters.
Karun, who is originally from Kerala, was taking part in a temple festival when the boat he was on capsized in river Pampa in Kerala.
"This is the best knock I've played in my life. There were many situations in the middle when I had to play differently, with (KL) Rahul, (Ravichandran) Ashwin and (Ravindra Jadeja) Jaddu. I have to thank them for supporting me, egging me on," Karun said at the end of the fourth day's play after India took a 282-run lead over England by amassing 759/7, their highest ever total in Test cricket.
"The first hundred is always important and I think when I got the first hundred, I didn't feel any pressure. I was just playing my shots after that," added the Karnataka youngster.
(With inputs from PTI)