"He was struggling, but that happens," Garnet Valley coach Rudy Shiller said. "I had confidence in him. He's our best hitter. That's why he bats third."
Andrew Bechtold (double), Steve Palis (RBI single), Shanfeldt (single), Chance Malek (RBI single), and Ben Fasco (single) immediately jumped on lefthander Rob Brown, giving the visitors a 3-0 advantage.
It was Brown's second start this season against the Jaguars.
"He works very, very fast," Shanfeldt said. "We wanted to slow him down. We wanted to take our time and hit the straight pitch."
Shanfeldt singled in each of his first two at-bats, doubled to right-center field in the fifth inning, and knocked in two runs with a single up the middle in a six-run seventh. He also drew a bases-loaded walk in the sixth.
The 17-year-old's only regret was that his parents, Robert and Rachel, were away in Chicago and missed his breakthrough performance.
"I'll text my dad and let him know," Shanfeldt said. "They'll be very excited for me."
Garnet Valley, improving to 9-3 overall and 8-3 in Central League action, stayed in the mix for top regular-season honors. Penncrest, Conestoga, Harriton, and Strath Haven are also in the logjam atop the standings.
"It felt like we lost three games today, but it was only one," Penncrest first-year coach Dan Sardella said. "We're right there with all the others."
Bechtold, a shortstop and Maryland recruit, went 2 for 4 with three runs, and Fasco, a rightfielder, was 3 for 5, including a two-run double to dead center in the seventh.
"Our first five guys set the tone," Shiller said.
Malek, who allowed two singles and one earned run in four innings, and fellow righthander Nick Bulisky (four Ks) combined on a five-hitter.
Said Shiller: "Chance was bringing it, throwing really well. Nick did a good job of getting ahead of batters."
For the junior-laden Lions (11-5, 9-3), outfielders Ron DiMatteo and Pete Muavero each went 2 for 3 with an RBI single.
Garnet Valley 302 012 6 - 14 16 2
Penncrest 100 101 0 - 3 5 3
WP: Chance Malek. LP: Rob Brown. 2B: GV-Andrew Bechtold, Jeff Shanfeldt, Ben Fasco.