Ada-Borup stunned two-time defending Class 1A champions Maranatha Christian Academy 70-53 on Thursday at Williams Arena.
The Cougars (27-3) slowly built their lead to 37-27 at halftime.
"I'd love to say part of our game plan was to get out to a 10-point lead, but we were just hoping to get it to a two-point lead," Ada-Borup coach David Smart said. "The girls just played well."
The Mustangs (26-5) struggled to catch up, playing significant minutes without two leading scorers in foul trouble. Senior center Onye Osemenam played only 13 minutes, fouling out with 5:06 remaining.
"Honestly, I do feel like it was kind of unfair, but we can't blame everything on the refs," Osemenam said. "We didn't play as well as we could have."
Mustangs coach Jim Hammond said he hadn't figured out what went wrong.
"We didn't play like we'd been there," Hammond said. "But I told the girls, you're trying to win three in a row. … You have not failed."
Senior forward Megan Kolness led Ada-Borup with 19 points and eight rebounds.