Scott Rolen led off the seventh inning with a homer that ended Zito's shutout, and the Reds rallied for a 4-2 victory on Wednesday night that left the Giants blaming themselves for the latest on a long list of losses in Cincinnati.
San Francisco has dropped seven straight and 12 of 16 at the ballpark. Zito remains winless here in six starts.
This one turned on a full-count pitch that Rolen hit for his first homer since July 6.
"I just didn't make that pitch to Rolen," Zito said. "That changed the whole inning. It shows you how quick this game can spiral."
It quickly spiraled out of control.
The Reds have won five of their last six overall.
Before the game, the Giants put Aubrey Huff on the 15-day disabled list while he gets treatment for an anxiety attack. Huff left the team after having a tough time in a game on Saturday. He could rejoin the team in San Francisco next weekend.
Zito said there was no indication that Huff was having anxiety problems.
"He keeps everyone loose in the clubhouse," Zito said. "He reminds us to have fun. I'm not quite sure what to make of that."
Pablo Sandoval singled in the third inning, giving him a hit in each of the first 18 games. That matches Johnny Rucker's mark with the 1945 New York Giants for best season-opening streak in franchise history.
Neither team did much through the first six innings, when a steady rain forced the grounds crew to spread bags of drying material around the infield at each changeover.
Angel Pagan hit a solo homer in the third off Bronson Arroyo, giving him a 10-game hitting streak that matches his career high. Arroyo lasted five innings, giving up at least one hit in each of them. He allowed nine in all and one walk, but escaped with only two runs -- one earned.
The Giants went only 1 for 6 with runners in scoring position, a recent theme. They're batting .156 with runners on second or third base in the last 10 games.
"No question that's hurting us," manager Bruce Bochy said. "We had some opportunities to cash in. We couldn't do it. In this ballpark, you're living on the edge with a two-run lead."
Until Wednesday night, Zito has struggled at Great American, going 0-2 in five starts with 7.20 ERA. The left-hander allowed only four singles through six shutout innings, then left after giving up Rolen's leadoff homer in the seventh on a full-count pitch that was over the plate.
Hensley, who hadn't allowed an earned run this season, gave up a single by Ryan Ludwick and set up the Reds' go-ahead runs with a slip. He momentarily lost his footing while fielding Ryan Hanigan's bunt and was off-balance when he threw wildly to first, failing to get an out.
"I just slipped," Hensley said. "The ball's wet. It's kind of a perfect storm.
"It's kind of pretty much all on me. In a situation like that, I need to reset my feet and get the out at first."
The Reds swept a three-game series from the Giants at Great American last July. ... Ryan Vogelsong starts the final game of the series for SF. He's 2-3 in his career against the Reds with a 5.40 ERA. ... Homer Bailey starts for the Reds, looking for his second straight win. He went a season-high seven innings in a 9-4 win in Chicago on Friday. ... The Giants filled Huff's spot by calling up infielder Joaquin Arias from Triple-A Fresno, where he batted .400. ... Sandoval hit in 22 straight games last season, a career best. ... Sandoval hit a foul ball that cleared the stadium roof on the first-base side.