After the visiting side won the toss, Karthik struck a watchful 105 off 225 balls with the help of 11 fours and a six.
Skipper Gokul Sharma contributed and unbeaten 59 off 103 balls, hitting 10 boundaries in the process at the Maharashtra Cricket Association Stadium.
During his knock, Karthik shared 75 runs for the fourth wicket with Amit Verma (32). He was the involved in an unfinished 130-run stand for the fifth wicket with Gokul as Assam continued their impressive run in the current domestic season.
But then followed the repair work that frustrated the hosts.
Both Fallah and Sanklecha picked up two wickets apiece.