At 18, Matthews is the youngest contestant, at least since 1937, to win the Patterson Cup and the Silver Cross, an award given for the lowest aggregate score in the Philadelphia Amateur qualifying and the Patterson Cup. A GAP official said records before 1937 are incomplete.
A GAP spokesman said its representatives met with club officials, who determined the golf course would be unplayable after the heavy rain. GAP's championship committee then decided to end the event after 18 holes, the first time it has been shortened by weather since 1994.
Matthews, who lives in Dupont in northeastern Pennsylvania and plays out of Fox Hill Country Club, was fine with the decision.
"I was really happy and thrilled," he said. "I'm blessed to have had an opportunity to do it."
Matthews' 6-under-par 65 was 1 stroke off the lowest score posted at the course since its renovation was completed in 2002. He posted three birdies and an eagle on the front nine and two more birdies and a bogey on the back.
Matthews said he had enough looks at birdies, and one at an eagle, to have brought a much lower score, possibly a 59, into play.
"That was the best I've hit the ball all summer," he said. "I probably only missed two shots all day. I was inside 15 feet for birdie on my first 14 holes, and I missed a seven-footer for eagle."
Four players tied for second at 68. They were Ben Cooley of Huntingdon Valley, Chris Fuga of Phoenixville, Michael Hyland of Little Mill, and Grant Skyllas of Moselem Springs.
Matthews, who won this year's Pennsylvania Junior Championship, said he was "extremely excited" to be starting at Temple later this month and playing for coach Brian Quinn.
"I've seen how good Coach has made people, and I'm grateful for the chance to work with him," he said. "I can't wait to play with all the guys on the team. They're great guys."
Contact Joe Juliano at 215-854-4494 or firstname.lastname@example.org, or follow on Twitter @joejulesinq.