Bundling out Services for a meagre 113, Assam ended the day two on 211 for eight with J Syed Mohammad unbeaten on 32 and Arup Das yet to open his account.
Assam have already extended their lead by 98 runs with two wickets remaining.
Earlier, resuming on 102 for eight, Services could add just 11 more runs to their overnight total, with Arup and Syed Mohammad accounting for the last two wickets.
In reply to Services' small total, Assam also lost their openers cheaply with scoreboard reading 17 for two before Roy and KB Arun Karthik (34) consolidated the innings.
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Although Assam also kept losing wickets at regular interval, Roy's innings of 81 off 192 balls, studded with six boundaries and two sixes, ensured that the hosts cross the 200-run mark.
Syed Mohammad also chipped in with crucial runs and remained not out till the end.
Services 1st inngs: 113 all out in 39.5 overs (Arup Das 5/40, Krishna Das 3/21).
Assam 1st inngs: 211 for eight in 85 overs (Sibsankar Roy 81, KB Arun Karthik 34).