According to Talk Sports 24x7, the Aussie will become the club's marquee player next season with the Indian side previously struggling in the final third.
Williams, 31, had a stellar season with the Nix scoring 11 goals across 27 A-League appearances and now looks poised to join Roy Krishna in departing the club.
ATK finished the 2018-19 ISL season in sixth and scored the second fewest goals in the competition.
The club have recently appointed a new coach in the form of Spaniard Antonio López Habas and have won the ISL twice in their history.
Williams' looming departure poses another headache for new Nix coach Ufuk Talay, who has lost a large chunk of the squad Mark Rudan was able to guide to the A-League finals.