The NL West-leading Giants lost for the 13th time in 17 games. Their once commanding 9 1/2 game lead has dwindled to two after losing for the 10th time in 12 home games.
Pinch hitter Tyler Colvin tripled to drive in the Giants’ first run in the sixth and later scored on Pence’s groundout.
Cueto faced the minimum over the first four innings. Pablo Sandoval reached on second baseman Brandon Phillip’s first error in 70 games and was erased on a double play in the second.
Madison Bumgarner (9-5) gave up a season-high five runs on nine hits, matching his season high. He walked one and struck out three.
Bumgarner needed just 53 pitches to get through four scoreless innings and then threw 26 in the fifth, allowing all five of his runs as the Reds sent 10 men to the plate.
Devin Mesoraco led off with a double, Brayan Pena grounded out to Bumgarner, holding Mesoraco at second. Chris Heisey singled and Zack Cozart singled to score a run. Cueto followed with a bunt, fielded cleanly by catcher Buster Posey. He threw late to third, loading the bases and opening the floodgates.
Hamilton, who had three hits, drove in two with a single, Todd Frazier, who had three hits, singled home a run and after Phillips hit into a fielder’s choice, Bruce singled in another run.
Bruce added an RBI single in the seventh against Yusmeiro Petit.