Two of their three hits in the fifth were swinging bunt-like singles by Brandon Nokes and Jeromy Hardy that loaded the bases after a single by Chaney Lum.
Adam Schofield, the winning pitcher, bounced out to National pitcher Brandon Burnett to score Lum to tie the game, 4-4.
Ryan Robinson followed with another bouncer back to Burnett, who looked Nokes back to third, but proceeded to throw wildly to first, allowing Nokes and Hardy to score. Robinson eventually scored when Patrick Hollis was hit by a pitch and Brandon Foote and Cody Palmer each walked.
"We got some breaks in the inning," said Scott Hardy. "We'll take them."
Burnett seemed to be in control on the mound after striking out the side in the third and getting two more of his 10 strikeouts in the fourth.
While South Mountain rallied, Schofield settled down on the hill. He allowed only one hit over the final three innings, a leadoff double in the fourth by Jordan Nicewarner. Pinch-runner Tre Hinchen scored what was the go-ahead run at the time.
"I was a little nervous in the beginning of the game," said Schofield, who struck out four and walked only one. "I just threw mostly fastballs and I started to settle down in the second inning."
Leadoff batter Nokes had three of South Mountain's six hits and scored two runs.
"You just never know what's going to happen until you play that first game," said National manager Jay Sutherland. "We lost the game on fundamentals."
Trevor Haupt and Hunter Phillips each had two of National's seven hits.