The Rebels (9-7) will next face Granada Hills Hillcrest Christian or Grace Brethren in a second-round contest at 7 p.m. Thursday at a site to be determined.
Beattie and the Rebels had to learn about the Cougars (13-3) on the fly. Glendale Adventist, which captured the Valley League championship, entered the match with an eight-match winning streak.
Flintridge Prep, which finished fourth in the Prep League, came prepared against the pesky Cougars.
"I had to consider it an advantage not knowing about them," Beattie said. "So we just assume we are playing somebody from our league,like Pasadena Poly or Chadwick.
"You have to prepare for the worst, and then only the best can happen."
The psychological ploy worked throughout most of the hour-long match.
Flintridge Prep junior outside hitter Mary McCluggage finished with a team-high nine kills and teammate Laurel Myers added eight. Senior setter Melissa Pearson registered 31 assists.
McCluggage and Myers attacked the net and kept Glendale Adventist, which was making its first postseason appearance since 2001, scrambling around the court.
"You have to go out there and play your game," said Myers, who is a junior middle blocker. "Our passing, hitting, setting and energy level were way up, and we got a lot of momentum winning the first game."
The Cougars, who were swept in each of their losses this season, showed marked improvement in the second game. Glendale Adventist Academy pulled to within 18-17 on a kill by senior middle blocker Kary Suarez.
Flintridge Prep then rattled off six consecutive points, highlighted by two kills by McCluggage.
The second game satisfied Glendale Adventist Academy Coach Bianka Tulgar.
"We pretty much played that second game the way we're used to seeing," Tulgar said. "We did a lot of things right.
"We were very nervous in the first game, and generally tired in the third game. We grew a lot throughout the season, and it's one of the best seasons we've had in the last five years.
"We were excited to make the playoffs and we got a home match out of it."
Glendale Adventist Academy junior outside hitters Cynthia Ramirez and Samantha Nicoletti each had three aces.
Ramirez contributed a team-best three kills and Suarez contributed two kills and five blocks.
Flintridge Prep recorded 14 aces, including five from senior outside hitter Erin Prickett. She also had four kills.
* CHARLES RICH covers sports. He can be reached at (818) 637-3228 or by e-mail at charles.rich@latimes.com.
