Rancho Cucamonga's Juan Melo, a thorn in the JetHawks' side when the teams battled for the Southern Division title in the first half, belted a key two-run double.
Todd Erdos, who saved the Quakes' crucial victory a week ago Thursday against the JetHawks at The Hangar, struck out the side in the ninth.
But a couple struggling Quakes - pitcher Kenny Henderson and catcher Ben Davis - stepped into the spotlight in Rancho Cucamonga's 6-4 victory in front of 6,325 at the Epicenter.
Henderson gave up two runs in five innings and Davis drove in two in the seventh with a single to even the first series of the second half at a win apiece.
"We hung a curveball to Melo with two outs," Lancaster manager Dave Brundage said. "That base hit Ben Davis got with two outs was the whole key to the ball game. That was the name of the game today - we couldn't take advantage of situations."
Trailing 6-2 entering the eighth, the JetHawks scored twice to cut into the lead.
Reliever Jim Baron walked Jason Cook and Jesus Marquez to lead off the inning and was replaced by Darell White.
After an out, Scot Sealy had an RBI groundout to make it 6-3 and James Clifford, who continues to haunt Rancho Cucamonga, stroked a run-scoring single to right to make it 6-4.
Gary Matthews Jr. made a diving catch on Chris Dean's blooper to preserve the two-run lead.
The Quakes took a 2-0 lead in the first.
Chris Prieto led off with a single and moved to second on a single by Rick Gama.
Both runners moved up on a double-steal with one out. Sean Watkins walked to load the bases, and Darryl Brinkley hit a sacrifice fly to center field.
Melo then hit a sharp grounder to the first baseman Clifford, who bobbled the ball and allowed Melo to reach first.
Clifford compounded his mistake by not paying attention to Gama, who never stopped running from second. By the time Clifford realized Gama was headed for home, his throw was late and the Quakes had an unearned run.
The JetHawks scored a run in the second on Dean's RBI-double that bounced beyond Prieto's glove on the warning track in center.
Shane Monahan tripled with one out in the third and scored on Cook's sacrifice fly as the JetHawks tied the game at two.
But the Quakes retook the lead in the bottom of the third on Melo's triple that went off the center field wall. Antonio Fernandez and Watkins both scored on the hit.
JetHawks starter Chris Beck, who will move back to the bullpen to make room for Greg Wooten in the Lancaster rotation, left after the fifth. He gave up six hits and struck out three.
Even though he gave up just three earned runs, Beck had trouble getting ahead of hitters. He threw 82 pitches, and 35 were balls.
Henderson, on the other hand, started strongly and finished well in his five innings. He struck out six and gave up four hits.
He retired the side in order in the first and fourth and even got away with a gaffe in the fifth that could have turned against him.
Mike Lanza led off the inning with a grounder down the right field line that was snared by Watkins at first. Henderson forgot to cover the base, though, and Lanza reached on an infield hit.
A vital sequence followed: Lanza was thrown out attempting to steal second by catcher Ben Davis and Monahan struck out.
Cook singled but Henderson struck out Marquez to end the inning.
Rancho Cucamonga loaded the bases in the seventh with two outs against Lancaster relievers Todd Niemeier and John Daniels and came away with two important runs.
Davis, one of the top prospects in the San Diego organization, hit a 3-2 pitch from Daniels into leftcenter field for a two-run single that made it 6-2.
Davis, 19, was hitless in his first three at-bats and entered the game batting .196.