Dustin King contributed a two-run triple in the first as Stamford took a 5-0 lead. Mattice drove a two-run homer to center field in the sixth.
Glen Cioccari had three hits and four runs for Stamford (5-0, 4-0), which visits Roxbury on Monday.
Michael Adlum went 2-for-4 for Jefferson.
Windham 22, Roxbury 3
Anthony Savasta doubled twice and drove in five runs to lead host Windham to a Delaware League victory.
Majd Ismail contributed two doubles, a triple and had three RBIs for the Warriors, who received two hits apiece from Aidan Cohane, Subaru Honge, Anthony Pettignano and Skyler Schoonmaker.
Tyler Phillips had two hits for Roxbury.
Downsville 10, Gilboa 6
Brad Dreher went 3-for-4 with two doubles and two RBIs to lead Downsville to a road victory in the Delaware League.
Austin Lamica contributed two hits and four RBIs for the Eagles.
Zach Perry had two hits and Hunter Grace allowed one earned run over four innings to lead Gilboa (4-2, 3-2), which travels to Hunter-Tannersville on Monday.