The Reds scored four times off young Mike Kickham in the second inning and Todd Frazier's 10th home run added three more in the third to build an early 7-0 lead.
Bronson Arroyo held off the slumping Giants, allowing a solo home run by Brandon Belt until the rains stopped play at the end of the sixth inning.
"The lead allowed me to relax and throw free and easy," Arroyo said. "With big hitter's up I didn't worry so much about putting every pitch on the corners of the plate."
After Kickham retired the side in the first inning, Brandon Phillips started and ambush with a single leading off the second frame. Jay Bruce followed with his 25th double, one shy of the NL lead. Todd Frazier drove Phillips home with a single. Zack Cozart hit a sacrifice fly to score Bruce. Ryan Hanigan singled. Arroyo moved the runners with a sacrifice bunt. Derrick Robinson hit a high chop over Pablo Sandoval's head at thirdbase to score two runs.
"We were more aggressive," Dusty Baker said. "Everybody got in on the attack. We needed that."
Joey Votto and Bruce singled in the third and both trotted home as Frazier's bomb landed 441 feet from home plate.
"I've been working on pitch selection," Frazier said. "I've been working on simplifying my swing. I've stopped wiggling the bat so much. I just put my foot down and go. He (Kickham) threw a fastball over the plate. It might be the only pitch you get to hit. You have to get on it early."
Frazier drove in just six runs in June but he drove in four on this, the first day of July.
"We had a little meeting and Dusty asked me, 'when was the last time we had a big inning,' " said Frazier who lost a card game to Johnny Cueto during the rain delay.
The Reds' reclaimed the run from Belt's home run in the bottom of the sixth. Phillips singled and Cozart's two-out double to leftcenter sent Phillips around with the Reds' eighth marker of the game.
The rain pelted the Great American Ball Park field for an hour and 28 minutes before the umpires put the Giants out of their misery with an 8-1 win. Fieldin Culbreth's crew called the game when the riverboat on top of the batter's eye started to list with the tide.
Arroyo gets credit for his 14th career complete game and the first shortened by rain. The Reds' right-hander improved his record to 7-6 on the season.
"I'll take it," Arroyo said. "It might be my only complete game this season. Twenty years from now on the back of my baseball card, it will be on my record and no one will have any idea (it was from a shortened game).