In December 2011, he won the Under 12 section of the World Youth Chess Championships held in Caldas Novas, Brazil.

[5] In 2014 he tied for third place at the Abu Dhabi Masters tournament[6] and helped the Indian team win the Under 16 Chess Olympiad in Győr, Hungary.

[8] In 2015, Karthikeyan won the 53rd National Premier Chess Championship of India in Tiruvarur edging out Vidit Santosh Gujrathi, whom he defeated in the direct encounter, on tiebreak, after both scored 8½ points from 13 games.

Though he had a slow start, and lost the match against eventual runner-up Aravindh Chithambaram, a crucial win against top seed Vidit Santosh Gujrathi on time control, followed by double withdrawals helped in gaining the lead and ultimately winning the championships.

[13] In June 2019, he placed second in the Asian championships,[14] aided by a win with black against Alireza Firouzja in which Karthikeyan sacrificed his queen on move 9.