ST. LOUIS -- Ryan Vogelsong wasn't supposed to start Tuesday night's game, but the San Francisco Giants are glad he did.

A late fill-in, Vogelsong authored six shutout innings as San Francisco blanked the St. Louis Cardinals 2-0 at Busch Stadium.

In improving to 9-8, Vogelsong allowed two hits, walked one and fanned five. After slogging through a 30-pitch first inning, the right-hander retired 16 of his last 17 hitters, allowing only a leadoff double by shortstop Jhonny Peralta in the fourth.

"After the first inning, my fastball command was pretty good," Vogelsong said. "I was a little erratic in the first. But after that, I was fairly decent. I feel like I've been throwing the ball really well in the second half."

Vogelsong knocked off Washington 3-1 on Thursday night, but San Francisco manager Bruce Bochy planned to start recent acquisition Mike Leake against St. Louis. However, Leake's hamstring didn't heal enough for him to come off the disabled list.

Bochy felt good about his plan B.

"Ryan's a warrior who gives you all he has, and that's all you can ask," Bochy said. "He was really on with the fastball command after the first inning, and then he started mixing in the curve and the cutter. He was locked in with his location."

Four relievers handled the last three innings for San Francisco (65-54), with Santiago Casilla garnering his 29th save. The Giants moved within 2 1/2 games of the first place Los Angeles Dodgers in the National League West, pending the outcome of the Dodgers' game in Oakland.

Lance Lynn (9-8) matched Vogelsong for five innings in a bounce-back outing. After giving up seven runs in the first inning Thursday night against Pittsburgh in his shortest major league start, Lynn delivered on his promise to improve.

He didn't allow a hit until shortstop Brandon Crawford's leadoff double off the glove of sprinting left fielder Brandon Moss in the fifth and took a one-hitter into the sixth. But San Francisco finally cracked the code, loading the bases with one out and scoring the one run it needed on Crawford's slow grounder to third that plated center fielder Gregor Blanco.

The Giants tacked on their last run in the seventh when first baseman Brandon Belt was plunked by reliever Randy Choate with the bases loaded. It scored Madison Bumgarner, who became the first San Francisco pitcher to pinch-hit successfully in 11 years with a two-out single.

Lynn permitted four hits and two runs over 6 2/3 innings, walking five and fanning six as the Cardinals (76-43) saw their NL Central lead over Pittsburgh fall to five games.

"I knew he wasn't going to put up a game like he had last time," St. Louis manager Mike Matheny said of Lynn. "He did what he needed to do and gave us a chance. But two hits in a game isn't going to cut it."

It was the ninth time the Cardinals have been blanked this year. They have scored just two runs in their last 21 innings, dating back to Sunday's 6-4 loss to Miami, and haven't produced a hit with runners in scoring position since Saturday night's 6-2 victory over the Marlins.

Meanwhile, the Giants got a win they had to have, thanks to the guy who wasn't supposed to start.

"This time of the year is fun," Vogelsong said. "It's like playoff baseball, even though it's August. I enjoy the intensity and the pressure."

NOTES: San Francisco RF Hunter Pence (left oblique) didn't start after injuring himself swinging at a Trevor Rosenthal pitch in the ninth inning on Monday night. ... St. Louis CF Jason Heyward (left hamstring) didn't start after leaving Monday night's game in the third inning, but was available to pinch-hit if needed. ... Giants RHP Mike Leake (hamstring) wasn't able to make the start as scheduled Tuesday night. Leake has allowed just two runs in 16 innings of two starts against the Cardinals this year.