Nair's knock of 111 was instrumental in Karnataka posting 179 for 9 in 20 overs after being sent in to bat by Tamil Nadu skipper Vijay Shankar.
Tamil Nadu was bowled out for 101 in 16.3 overs. No other Karnataka batsman crossed 20 even as Nair, who opened the innings, smashed eight fours and eight sixes in his 52-ball effort.
The only significant partnership was the one between Nair and R Samarth for the third wicket, which yield 83 runs. The centurion kept going despite the team losing wickets at regular intervals.
Rookie slinger Athisayaraj Davidson with a five-wicket haul played a huge role in restricting Karnataka to 179 after they looked set for a big score at one stage.
Nair was among his victims as he ran through the lower order to finish with 5 for 30.
Tamil Nadu was in early trouble during the chase, losing opener Abhinav Mukund (1) and the prolific Dinesh Karthik (0), who had scored three successive fifties, to a run-out in the first two overs.
Despite a 48-run partnership between M S Washington Sundar (34) and Shankar (20), things went downhill for Tamil Nadu after they were separated.
Dubey, who ended Sundar's stay, took three other wickets including that of the aggressive N Jagadeesan (16) to send Tamil Nadu hurtling to defeat.
Meanwhile in matches played at Visakhapatnam, Andhra Pradesh beat Hyderabad by six wickets riding on Ricky Bhui's 73, while Kerala outplayed Goa by nine wickets.
Three teams - Karnataka, Tamil Nadu and Andhra - have 12 points after four games while Hyderabad has 8 and Kerala (4).
Goa has lost all its matches.
Brief scores: Karnataka 179 for 9 in 20 overs (Karun Nair 111, Athisayaraj Davidson 5 for 30) beat Tamil Nadu 101 all out in 16.3 overs (M S Washington Sundar 34, Pravin Dubey 4 for 19). Points: Karnataka: 4; TN: 0.
At Visakhapatnam: Hyderabad 129 for 9 in 20 overs (B Sandeep 39, Harishankar Reddy 3 for 30) lost to Andhra 133 for 4 in 18.2 overs (Ricky Bhui 73, Mohd Siraj 3 for 26).
Points: Andhra: 4; Hyderabad: 0.
Goa 138 for 8 in 20 overs (Keenan 36, K M Asif 3 for 25, Abhishek Mohan 3 for 30) lost to Kerala 140 for 1 in 15.5 overs (Sanju Samson 65 not out, K B Arun Karthick 37 not out). Points: Kerala: 4; Goa: 0
First Published: January 12, 2018, 6:01 PM IST