Freshman Anna Ross earned 18 points, calmly handling the ball and minimizing mistakes against the Spartans' pressure, while Sam Peebles added 10 points and Maggie Tripodi got six points.
Maddy Wetterhahn, in defeat, led South Jefferson with 24 points as Rachel Bassett added 14 points, but Arin Crowell, the Spartans' top outside shooter aside from Bassett, made just one 3-pointer all afternoon.
To win it all, Westhill will need to spring more surprises - including in the Class B semifinal against no. 5 seed Utica-Notre Dame, as they square off Tuesday at 7:45 at Vernon-Verona-Sherrill High School.
The Jugglers, who won the sectional Class C title a year ago before moving up in rank, have smashed Canastota (85-51) and Ilion (88-60) so far in the playoffs, sporting a high-octane offense led by freshman sensation Emily Durr, who had 35 points against Canastota and 33 against Ilion.
Pull off this upset, and Westhill will meet someone very familiar - Cazenovia or Jordan-Elbridge - in next Sunday's Class B final at 1:45 at Utica Memorial Auditorium.