SAN DIEGO -- San Diego's bullpen gave the Padres a chance, and they made the most of it.
"We just kept pecking away," San Diego manager Bud Black said. "And the pitchers just kept the momentum on our side."
Nate Schierholtz hit a three-run homer in the first, but Chicago's lineup stalled after the fast start.
"They beat our butts the last eight innings," Cubs manager Dale Sveum said.
It was a remarkable turnaround from the start of the game, when San Diego appeared to be headed for a long night when the first eight batters reached against Edinson Volquez.
Leadoff batter Starlin Castro reached on a throwing error by third baseman Headley, moved to second on Anthony Rizzo's single and scored on Dioner Navarro's base hit. Schierholtz then connected for his 19th homer to make it 4-0.
Brian Bogusevic added a two-run triple and Volquez was lifted after only recording two outs, matching the shortest start of his career. The right-hander was 5-0 with a 2.76 ERA in seven career starts against the Cubs.
Tim Stauffer came in and pitched 4 1-3 innings of one-hit ball. The Padres' bullpen threw 8 1/3 shutout innings in the win.
"It was really Tim keeping us in the game and the bullpen doing a great job," Venable said. "The positive thing was there was still a long game to go and we were able to crawl back in there."
San Diego responded with three in the fourth on Gyorko's 15th homer, a drive to center on a 1-1 pitch from Edwin Jackson. Venable added a two-run triple of his own in the fifth, and the Padres were right back in the game.
Venable went 3-for-4 and is batting .373 (31-for-83) with six homers and 12 RBIs in his past 21 games. His 18th homer traveled an estimated 441 feet into the right-field seats.
"That was one of the longest ones of the year," Black said.
"He's swinging at strikes and taking balls, as simple as that sounds. He is putting good swings on the ball."
Forsythe came up with a tying triple in the seventh, and Venable put San Diego ahead to stay with his shot off James Russell (1-5). Gyorko greeted Blake Parker with a drive to right on his first pitch of the eighth, providing an insurance run.
"It's not the ideal start, but there is nine innings of baseball left and anything can happen," Gyorko said. "We had great at-bats up and down the lineup."
Jackson allowed six runs, five earned, and 10 hits in six-plus innings. He is winless in his past four starts.
"Your team goes out and gives you six runs and any starter that we have, we all feel the same way. If you can't hold a six-run lead, it's real disappointing," Jackson said.
Black said he sensed the Padres would come back when Stauffer was greeted at the mound by catcher Nick Hundley.
"When we made the pitching change Nick was the first guy to say, `Hey, we got nine at-bats," Black said. "All the (infielders) had a good look in their eye."
NOTES: Rizzo batted second in the order for the third straight game, a spot he hadn't hit in before Wednesday, when he slugged two home runs. ... The Cubs go against a left-handed starter in the Padres' Eric Stults (8-10) on Saturday. It breaks a 15-game streak of facing a right-handed starter. ... Padres OF Carlos Quentin will test his balky right knee by running on Saturday. Quentin has been on the DL since July 31. Manager Bud Black said if the news isn't positive regarding Quentin, an operation is a possibility. ... Padres OF Kyle Blanks will be sent out on a rehabilitation assignment on Saturday. Blanks had been sidelined by a left Achilles injury; he's been on the DL since July 12. Blanks will go to Triple-A Tucson or Class-A Lake Elsinore.