CHICAGO -- Ryan Dempster's new, authoritative approach has paid off.
"I think I'm probably a lot more aggressive," said Dempster, now 2-4 and with two wins in his last three starts. "For some reason, it felt like before I was trying not to lose the game rather than win it.
"I don't know the reason why and I don't really care what the reason is, but I went there today and attacked the strike zone and got after guys."
Dempster (2-4) allowed two runs and six hits in six innings and the Cubs ended San Francisco's six-game winning streak.
Giants left-hander Madison Bumgarner (0-6) gave up two earned runs on five hits through six innings, struck out six and walked two. He hasn't won since Sept. 30, 2010.
"He's throwing the ball well enough to get a win," San Francisco manager Bruce Bochy said. "But we're just having a hard time finding a way to get him one."
Cody Ross hit a two-run homer in the eighth that cut the margin to 8-4. But in the bottom half, Chicago added three more runs on Castro's double, Barney's single and Johnson's two-out blooper to left field.
Dempster struck out Ross his first two times up.
"He was really good today, getting his slider over for a strike." Ross said. "It wasn't like he was aiming it. He was putting it in a good spot. He kept throwing it and throwing it."
Aubrey Huff drove in the Giants' first two runs, on a single in the first inning and a double in the fifth.
In between the Cubs scored three runs in the bottom of the second. Koyie Hill had a sacrifice bunt, and then came home on Castro's single with two outs. Barney added a single.
A first-inning infield single extended Byrd's hitting streak to 14 games. ... Until Friday, each of the Giants' previous 19 games had been decided by three or fewer runs, and they led the league in one-run games with 15. ... Saturday's night game against the Giants will be the first Saturday night game at Wrigley Field in nine years. ... Cubs right-hander Randy Wells, on the disabled list with a forearm strain, is scheduled to make a rehab start against Quad Cities next Tuesday at Class A Peoria. ... Giants right-hander Ryan Vogelsong (2-0) will make his fourth start of the season on Saturday night against left-hander Doug Davis, who is making his first start for the Cubs after being promoted from Triple-A Iowa.