Arroyo (1-0) retired the first 14 Giants batters before yielding his only hit -- a fifth-inning single to Brandon Belt. The right-hander allowed one walk and struck out four en route to his first career post-season victory.
Ryan Hanigan went 2-for-4 with three RBI and Jay Bruce ignited a five-run eighth inning with a two-run double for the National League Central champion Reds, who pushed the Giants to the brink of elimination in the best-of-five NLDS.
Joey Votto added three hits and scored two runs for the Reds.
Madison Bumgarner (0-1) was touched for four runs on seven hits in a 4 1/3-inning effort for NL West champion San Francisco, which now must go to Cincinnati and win three straight to advance.