Sandoval's sacrifice fly in the fifth inning extended the Giants' lead to 5-2 and he capped the scoring with a mammoth, two-run homer in the seventh as San Francisco forced a fifth and deciding game in the NLDS.
Angel Pagan also went deep and added a run-scoring double, while Gregor Blanco clubbed a two-run shot as the Giants banged out 11 hits in the victory.
Tim Lincecum (1-0) hurled 4 1/3 innings of one-run ball after coming in as a reliever to earn the win for the NL West champions, who took a second straight in Cincinnati after dropping the series' first two contests at home.
Mike Leake (0-1) -- who started in place of injured Reds ace Johnny Cueto -- was roughed up for five runs on six hits in a 4 1/3-inning effort.
Ryan Ludwick socked a solo homer for Cincinnati, which was backed into a winner-take-all contest Thursday at Great American Ball Park.