Eliza Stein headed in a corner kick by Kendall Sniper, Lucy Tidd stopped a penalty kick in the second half, and the ninth-seeded Bulldogs (8-7) held on to beat the eighth-seeded Spartans (7-6-2) in a Class A South prelim in Sanford.
Sniper lofted a well-placed service in front of the goal and Stein popped it into the left corner under the crossbar 17:34 into the game.
Sanford was awarded a penalty kick with 15:23 remaining when Hailey Tarbox took a well-timed through ball from Ava Hudson into the box and was fouled. Hudson took the penalty, which Tidd stopped while on sliding on her knees.
Portland advances to face No. 1 Scarborough in the quarterfinals Tuesday night.