At stumps on the third day at the PCA Stadium, Karnataka were 351/5 in 110 overs with 81 runs ahead of the hosts in the first innings.
Nair was batting on 107 with Amit Verma, playing his first game of the season, on 65 not out. The duo has so far added 110 runs for the sixth wicket.
After bowling Punjab out for 270 on Sunday, Karnataka drove home the advantage today with Manish Pandey too contributing 65. Wicketkeeper-batsman CM Gautam made a quick 48 off 65 with 7 fours.
The right-handed Nair has been in great form in the last three matches. In his short first-class career he has three tons in successive matches. He has joined Brijesh Patel, Roger Binny and Rahul Dravid in achieving this feat for the state.
The 22-year-old's sequence of centuries started against Delhi when he scored 105. He followed that up with 100 in the quarter-final against Uttar Pradesh.
Today's 191-ball knock with 9 boundaries will be the most cherished by the young man considering this is the semi-final.
Left-handed Verma, after a cautious beginning, settled to play the perfect supporting role to Nair. Verma is unbeaten on 65 off 152 balls with 11 boundaries.
Pandey's 65 came off 131 balls with six fours and a six. Opener Robin Uthappa hit 47.
Harbhajan Singh, who has beeen out of the Indian team, bowled 24 overs and returned with figures of 2/79.
Karnataka 351/5 in 110 overs (Karun Nair 110 batting, Amit Verma 65 batting, Manish Pandey 65, Robin Uthappa 47, CM Gautam 48, Harbhajan Singh 2/79) Vs Punjab 270.