Samarth remained unbeaten on 118 off 276 deliveries with the help of 10 boundaries, while Nair's fifty came off 168 balls and five fours as Karnataka reached 248 for three in 90 overs at stumps on the opening day.
Electing to bat, Karnataka lost opener Mayank Agarwal (15) and Robin Uthappa (2) early before Samarth and Nair shared a 155-run third wicket stand to take Karnataka to a comfortable position.
Medium pacer Ashish Kumar (3/47) picked up all the three wickets for Jharkhand.
Brief Scores:
Karnataka: 248 for three in 90 overs (Ravi Kumar Samarth 118 batting, Karun Nair 74; Ashish Kumar 3/47).