Teesdale, who began his afternoon round on the back nine, was tied with McDermott at 5-under going into his second nine, then birdied Nos. 1, 2, and 3 to build a 3-stroke margin. He parred his next five holes before coming to the 148-yard ninth with the threat of a thunderstorm imminent.
He pulled his tee shot left down a hill, hit a lob wedge just off the green, stubbed a putt from the collar, and propelled a 25-foot bogey putt four feet past before finally securing the victory.
"I wasn't distracted" by the threatening weather, Teesdale said. "I just hit bad shots, especially the tee shot."
As for his thought on the final putt: "Just make it. I just had to get it on line because it was dead straight and the greens were rolling well."
McDermott, seeking to become the second player to win all four GAP majors, followed a 67 in the morning with an afternoon 70 for 137. Rich Steinmetz, head professional at Spring Ford, shot rounds of 69 and 71 for a 140 to finish as low pro and pocket the top check of $7,000.
Teesdale, a member at Commonwealth National and a graduate of Hatboro-Horsham High, said he appreciated the support he received, most via texts, from his Temple teammates and coach Brian Quinn.
"They've been unbelievable," he said. "I actually played with Brandon last night at Commonwealth. It's good to have someone push you like that. He and Mason are great players. Mason texted me [Tuesday] night and said, 'Play smart, play your game. Don't force anything. It's a long day, 36 holes, so don't get upset over one shot.' "
McDermott, a five-time player of the year who has won seven GAP majors but not the Open, hurt himself with a pair of 3-putts on his final nine but gave credit to Teesdale.
"There was a point when it seemed like my tournament, and he just played spectacular golf and made it his tournament," McDermott said.