The Spartans (15-11, 7-3 in league) came out guns blazing as every shot seemed to fall for them in the first quarter en route to building a 24-8 lead on a three-pointer and two made free throws from Courtney Chen six minutes and 34 seconds into the game.
La Cañada's offense suddenly went cold as it only managed 19 points in the remaining 25 minutes. The Spartans also left 11 points at the line, shooting 35% (six for 17) at the free-throw line.
"When we miss 11 free throws and it's a three-point game with a minute left, those come back to haunt you," Hill said. "It's tough, it's one of those tough losses."
The Wildcats (11-5-1, 8-1), on the other hand, came alive and went on an 18-4 run that started at the first and spanned to the middle of the third quarter when Monrovia's Taylor Burton capped the run with a free throw, cutting her team's deficit to 28-26 with four minutes left in the third quarter.
The Spartans maintained a one-possession lead, 33-31, going into the final quarter behind a bucket each from Chen, who finished with a team-high 20 points and eight rebounds, and Lauren Barsamian in the final four minutes.
The fourth quarter was tightly contested, featuring five lead changes and one tie.
Chen tried to hold off Monrovia as she hit a three-pointer with 5:40 remaining to give La Cañada a 40-38 lead.
The Wildcats locked down on defense from there and scored three buckets in a row to take a 46-41 lead on a corner jumper from Domynyque Johnson, who had a game-high 21 points, after a Spartans turnover with 3:06 to go.
Chen cut the lead to three, 46-43, when she hit a running jumper with 2:10 to go. She looked to tie it with 27 seconds left on a three-pointer, but the shot was off the mark and rebounded by Johnson.
La Cañada quickly fouled Monrovia's Gina Henderson, who had six of her nine points in the fourth quarter, but she made both of her free throws to ice the game.
"We held [Henderson] to nine points and that's our game plan but unfortunately, we couldn't get it done on the offensive end," Hill said. "The past few games the fourth quarter has been our best, but we didn't have that same execution tonight."
Keilee Bessho had nine points and five rebounds for the Spartans, who also got nine rebounds from Sydney Zarate.