Resuming at overnight 212/4 in reply to Rajasthan's first innings total of 186, Assam's middle order, spearheaded by Karthik, put up a splendid performance to give the home team complete command of the match.
Karthik shone the brightest among Assam batsmen with his 315-ball 151, which included 12 hits to the fence and three over the boundary.
Later, Swarupam Purkayastha (47) and Kunal Saikia (50 not out) also made noteworthy contributions to pile on the agony for Rajasthan bowlers.
In reply, Rajasthan had their back against the wall after being reduced to 47/3 with Krishna Das scalping two wickets.
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Brief Scores:
Assam: 422/9 declared in 151 overs (K B Arun Karthik 151, Swarupam Purkayastha 50, K Ajay Singh 5/116).
Rajasthan: 186 and 47/3 in 15 overs (Ashok Menaria 20 batting, Krishna Das 2/13).