Cubs reliever Pedro Strop (1-0) escaped a bases-loaded, none-out jam in the bottom of the eighth, sending the game into the ninth as a scoreless tie.
Sergio Romo (3-6) took the mound for San Francisco, and after striking out Starlin Castro, delivered a 3-2 pitch to Schierholtz that the ex-Giant sent into the right-center field bleachers for his 14th homer of the season.
Chicago starter Chris Rusin hurled seven shutout innings, scattering three hits while walking two and striking out three.
Madison Bumgarner fired eight scoreless innings, yielding just four hits as San Francisco dropped a third straight overall.
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