Chasing 165 for a win, Assam, who started the fourth and final day on 70 for three, achieved their target in the second session with man-of-the-match Arun Karthik unconquered on 55. Delhi slumped to their first loss of teh season.
The victory though was set up by their bowlers, who bundled the visitors for 149 after putting the Gautam Gambhir-led side into bat.
New-ball bowlers Krishna Das (4-38) and Arup Das (3-49) shared seven wickets each to rattle the Delhi top-order.
Delhi bowlers Manan Sharma (4-41) and Pradeep Sangwan (3-33) made it count at the Barsapara Cricket Stadium.
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But Delhi's second batting outing was also a short affair after the Assam bowlers once again made life difficult for the visiting batsmen, who could only muster 172.
Even though Unmukt Chand scored a gutsy 44 in the second innings, Gambhir, who managed 4 and 0 in the first and second innings respectively, failed to lead by example.
Left-arm spinner Jamaluddin Syed Mohammad and leg-spinner Amit Verma bagged three wickets each, while Krishna and Gokul picked two apiece.
Assam are closing the gap on Delhi (24 points in 7 matches) with 22 points in 6 games with third-placed Vidarbha also having the same number of points but a game more in their bucket.
Two more rounds to go and both the teams have put themselves in a strong position to secure a quaterfinals berth from Group A.
Brief Scores:
Delhi 149 and 172
Assam 157 and 168/5 in 56 overs (Rahul Hazarika 59, Arun Karthik 55 not out; Manan Sharma 2-51).