Waltiea Rolle added 10 for the third-seeded Tar Heels (27-5), who advanced to Saturday’s semifinals to face 10th-ranked and second-seeded Maryland.
Nicole Boudreau scored 15 for the Eagles (12-19), the second No. 11 seed to win a tournament game with its upset of sixth-seeded Virginia in Thursday’s first round.
UNC won both regular-season meetings by 28 points apiece, but this one stayed tight throughout. The Tar Heels led 54-53 with 3 1/2 minutes left before Ruffin-Pratt scored six points—including when she muscled up a drive over Kristen Doherty with 38.7 seconds left to make it a two-possession game—to help UNC finally stretch its lead.
Ruffin-Pratt also hit 11 of 12 free throws.
BC shot 31 percent (18 of 59) but hit 10 3-pointers.