Flintridge Prep (7-1) got all it could handle from the Gauls (8-2, 1-1 in league). Webb went 0-11 in league last season, but didn't come close to resembling that Thursday.
The Rebels, who are seeking their first league championship since 1996, received 23 digs and 42 assists from setter Casey Meurer and 11 kills and five aces from outside hitter Julia Miller.
"The league is tough for sure, and it's always going to be tough," said Meurer, who had a pair of aces in the fifth game. "I think we have a great chance to do well in league.
"There's just a lot of solid teams, and you expect the best from each team in each game. You can't go in thinking any one match will be easy."
Two Prep League teams have combined for four consecutive CIF championships. Westridge won a crown in 2006, followed by Mayfield the past three seasons.
The Rebels and Gauls took their share of turns riding waves of momentum.
Webb overcame a 20-14 deficit in the fourth game by going on an 11-1 run to force a deciding game.
"Going into the fifth game, we talked about getting it going and talking it up more," Meurer said. "We looked at it like it was a fresh start."
Flintridge Prep raced to a 7-1 lead in the finale before Webb closed to within 8-5. Two straight aces from Meurer made it 11-5. However, the Gauls pulled to within 12-11 before the Rebels picked up the next three points. An ace by Charlotte Kay closed out the victory.
The Rebels, who will meet Webb again on Oct. 19 at Webb, got 29 digs from Tori Glebocki, six aces and five kills from Kate Woolley and 10 kills from Kelly Wang.
In the first game, Flintridge Prep jumped out to a 10-2 lead on a kill by Kay. It concluded the opening game with two aces in a row from Glebocki. Flintridge Prep held a 23-22 lead in the second game, but three straight points from Webb deadlocked the match at one game apiece.