SAN FRANCISCO -- In a San Francisco Giants season reduced to a series of silver linings, left-hander Ty Blach has gone above and beyond.

Basically, he has been golden.

Supported by a first-inning, three-run homer from Buster Posey, Blach gave San Francisco another strong seven innings as the Giants beat the Chicago Cubs 6-3 at AT&T Park on Tuesday night.

"The last couple outings we've had some early leads for me, and it makes it fun to go out and pitch," Blach said. "I'm just trying to execute pitches. It's been fun."

Despite losing for only the second time over its past 14 road games, Chicago maintained a 1 1/2-game lead in the National Central over Milwaukee, which lost at Minnesota.

"We're playing like that .500 team that we were in the first half, and I don't like it," Chicago manager Joe Maddon said. "We've got to get sharper."

Sharp was not a problem for Blach (8-7), who allowed two runs while scattering seven hits and striking out three. With 100 pitches thrown upon exit, he went at least seven innings for the fifth straight game.

And the 26-year-old did something few left-handers have accomplished against the Cubs this season -- he beat them. Chicago is 19-8 against southpaws, the best mark in the majors.

"When a major league pitcher commands the ball well -- both sides -- has a good changeup, a breaking ball to go with it and just locates well with good movement, he's got good poise out there," Giants manager Bruce Bochy said. "He did a good job to minimize damage. They got a few hits off him, but he did a great job keeping it to one run, and that's what he's good at."

Reliever Sam Dyson threw a scoreless ninth, earning his seventh save in eight tries since joining the Giants.

Trailing 4-1 after five innings, the Cubs scored in the sixth on back-to-back doubles by Willson Contreras and Ian Happ. The Giants retaliated in the seventh on Joe Panik's fielder's choice grounder to plate Kelby Tomlinson.

Happ delivered a sacrifice fly in the eighth, but San Francisco's Brandon Crawford singled to score Posey to reset the three-run margin.

The Giants broke on top when Posey launched his 12th homer of the season into the first row of the left field bleachers.

"It was nice to get on the board against a good pitcher," Posey said.

Giants ambushed Chicago starter Jose Quintana (6-10 overall, 2-2 with the Cubs) at the outset as four of their first five batters reached base. Gorkys Hernandez slapped a first-pitch leadoff single to left. Hunter Pence reached one batter later when shortstop Javier Baez booted a double-play grounder.

Posey picked on Quintana's first offering to make it 3-0 on a play that stood following review as his home run ball caromed off the hands of a fan not judged to be reaching over the fence.

"Yes, it was very frustrating," Quintana said. "First inning was a tough inning for me."

Blach's two-out, two-strike single to right plated Crawford in the fourth for a 4-0 lead.

It marked the second straight start Blach has contributed at the plate. He hit the team's first three-run homer at home this season in his last start on Thursday.

"You've got be able to do all aspects of the game," Blach said. "I was lucky to be able to flair one in there today."

The Cubs broke through in the fifth when Jon Jay's infield out enabled Jason Heyward to score from third.

NOTES: The Cubs' streak of having allowed three or fewer runs on the road ended at 11 straight games, the longest such streak in the majors this season, according to STATS. ... 2B Joe Panik replaced Giants rookie Miguel Gomez in the top of the second inning. Gomez aggravated his knee running out a ground ball to short during his only at-bat in the first. Bochy said afterward Gomez was removed as a precaution and is day-to-day. ... Cubs 1B Anthony Rizzo celebrated his 28th birthday by going 1-for-4 with a fourth-inning single. ... Chicago RHP Koji Uehara left the game with a trainer after facing two batters in the seventh. Chicago manager Joe Maddon said afterward that Uehara had a stiff neck.