The seventh-seeded Lady Warriors led 33-30 with 6:20 left in the game, when 10th-seeded Salem pulled ahead to stay.
Abby Blevins (eight points) jump-started the Lady Hounds' comeback with a big-time 3-ball that locked the score at 33-all.
Westside broke the tie with an inside hoop from Lauren Hogue, but next time down court for Salem, Blevins was at it again and drilled another triple to ease the Lady Hounds in front 36-35.
Then for good measure, Sidney Mitchem racked up an old-fashioned 3-point play, followed a minute later by a charity toss, to give Salem a 40-37 lead. Mitchem paced Salem with 15 points on the afternoon.
Salem extended its cushion to 46-39 on a layup from Paige Hollis (six points) and two free throws by Blevins with 45 ticks left on the clock.
Kaly Hogue hit a late 3 for Westside, but Whitney Ward (12 points) wrapped up Salem's victory with a layup near the buzzer, giving the Lady Hounds a seven-point win in the girls' Division II brackets.
Hollis opened the game with a trey for Salem, but Westside used a 7-0 run to grab a 7-5 advantage after one.
Salem opened the second period strong, scoring the first eight points of the frame to take a 13-7 lead.
Then it was Westside's turn, with the Lady Warriors reeling off eight unanswered points to slide on top 15-13.
Buckets from Dalina Perkey (five points) and Ward inside the last minute of the first staked Salem to an 18-17 lead at intermission.
Westside fought back in the third and took a 31-28 lead at the end of the third period, before Salem rallied in the fourth to post a 50-43 victory.
Hailey Crisp finished with four points for the Lady hounds.
Heidi Taylor paced Westside with nine points.
In opening-round action Dec. 23 in the boys' Division II brackets, Class 4A Valley View, seeded third, pushed past the 14th-seeded Salem Greyhounds 62-40.
The Blazers led 16-6 after one, 34-18 at the half and 48-30 after three periods of play.