Of the season-high four errors San Francisco committed, three of them were induced by the Reds' quickness on the basepaths.
Cincinnati leadoff batter Billy Hamilton stole two bases while forcing wild throws from catcher Hector Sanchez and starter Tim Lincecum.
"It's fun being fast," said Hamilton, who has 22 thefts in 28 attempts this season.
Sanchez committed a second throwing error, and Lincecum permitted Brandon Phillips to steal third base so easily that he was halfway to his destination before the right-hander released his pitch. The Reds finished with four steals, equaling the most by a Giants opponent this year.
Before the game, Giants manager Bruce Bochy explained that a catcher shouldn't try to neutralize a speedster such as Hamilton by overthrowing.
"Get rid of it," advised Bochy, a former catcher. "Don't try to do it with the throw. Try to do it with the release."
Sanchez understood that concept. But he reminded interrogators that many athletes ignore a controlled approach under the influence of competition.
"I tried to be quick, tried to do more than I could. That happens, you know?" Sanchez said. "Game action is different. You want to get the guy out."
The Giants lost only their fourth game in 12 starts by Lincecum (4-4), who matched a career-high by yielding eight runs in 4 1/3 innings. Cincinnati happened to be Lincecum's antagonists the last time he allowed that many runs on July 22, 2013. The Reds remain the lone National League team that Lincecum has not defeated in the regular season during his eight-year career.
"I didn't do a good job of focusing from batter to batter," Lincecum said with a heavy sigh. "Hamilton was a big part of their offense, getting on base. ... He flustered me a little bit."
Lincecum's ERA climbed to 5.01, reflecting his inability to excel in back-to-back outings. He remained confident that he ultimately will find consistency.
"A lot of people think that there's a big space between being good and not being good," he said. "I think it's a small fraction of space to make up."
The Giants still own the Majors' best record (37-21), but that status might be endangered if they played here more frequently. Since 2010, they have dropped 11 of 12 regular-season games at Great American Ball Park while being outscored 73-30.
The Reds scored in only two innings, but amassed four runs each time.
Hamilton established the tempo immediately by doubling to open the Reds' first. Lincecum caught him leaning the wrong way but short-hopped his pickoff throw past second baseman Joaquin Arias. Hamilton was credited with a steal of third and scored on the first error of the evening.
Bochy downplayed the impact of Hamilton's speed on that play, noting that Lincecum didn't have to rush his throw.
"I see that happen a lot," Bochy said. "A guy does a spin move and they're not expecting to [trap the runner]."
Todd Frazier walked and scored on Jay Bruce's double -- his 500th career RBI -- before Devin Mesoraco lashed a two-run homer.
Led by Hunter Pence, who homered in the first inning and lacked a triple to hit for the cycle, the Giants trimmed the difference to 4-3 before Cincinnati struck again in the fifth.
Hamilton's infield single began the uprising. He stole second base, proceeded to third on Sanchez's throwing error and scored along with Frazier on Phillips' double. Phillips executed his uncontested steal of third before scoring on Bruce's single. Bruce reached third on a steal and Sanchez's accompanying misfire, then scored on Brayan Pena's fielder's-choice grounder.