Having been named man of the series in New Zealand’s 3-0 whitewash against India, Will Young added just 19 from 19 balls, as his teammates fell around him.
Regular strikes from Sri Lanka meant the Black Caps’ highest partnership was 39 runs, coming from Foulkes and Ish Sodhi (10).
Despite Santner striking in his first over, removing Kusal Mendis LBW for a duck, the Black Caps were always going to struggle to defend their under-par total.
Stands of 40 runs between Pathum Nissanka (19) and Kusal Perera (23) and 38 between Asalanka and Wanindu Hasaranga (22) chipped away at the bulk of the target, as New Zealand ultimately had too small a target to defend.
Backing up his 16-ball innings, though, Foulkes did stand out with the ball as well.
After bowling Nissanka at the start of his spell, Foulkes returned to remove Bhanuka Rajapaksa for four – and give debutant wicket-keeper Mitch Hay his first catch in international cricket – and Hasaranga, caught by Santner at midwicket.
However, despite a late scare when Hasaranga departed in the 18th over, Asalanka held his nerve, supported by a seven-ball 11 from Dunith Wellalage to guide the hosts home.
The second and final game of the series takes place at the same venue on Monday morning (NZ time).
New Zealand 135 all out (Foulkes 27 not out; Wellalage 3/20).
Sri Lanka 140/6 (Asalanka 35 not out; Foulkes 3/20).
Alex Powell is an Online Sports Editor for the NZ Herald. He has been a sports journalist since 2016, and previously worked for both Newshub and 1News.