CHICAGO -- Jason Hammel wasn't happy to be pulled after walking two runners to open the fifth inning in Thursday's wild card showdown with the San Francisco Giants.

But the Chicago Cubs right-hander said he understood the rationale of manager Joe Maddon who didn't want a 5-2 lead to slip away.

"I felt like I had earned the right to get out of that situation," said Hammel, who worked four-plus innings and had no decision. "As a competitor I want to be out there and clean up my own mess.

"But I was able to talk with Joe, turn the page and the bottom line is we won the ballgame."

Right-handed deliver Justin Grimm (2-3) shut down that threat with a perfect fifth and the Cubs (59-48) held on for a 5-4 victory.

They also leapfrogged over the Giants (59-49) into the second wild card spot after the opener of a four-game series.

"(Hammel) had really good physical stuff from where I was sitting but the command was off a little with the fastball," said Maddon. "I did not want to let them back into that game right there. ... I knew the bullpen was rested, I knew we had the guys to do it."

Rookie catcher Kyle Schwarber's first career Wrigley Field home run helped lift the Cubs to an early lead on the way to the victory.

"I knew (Giants starter Chris Heston) had some good sink on his ball and I was just trying to get a pitch I could drive," said Schwarber, who went 1-for-3, walked, scored twice and collected three RBIs. "I got myself into a decent count and put a good swing on it and it went."

Grimm (2-3) worked the perfect fifth inning to earn the victory.

Chicago led 5-0 after two innings, but two-run homers by San Francisco first baseman Brandon Belt and shortstop Brandon Crawford sliced the margin to one. However, the Cubs' bullpen blanked San Francisco over the final three innings.

Heston (11-6) took the loss after a four-inning outing.

Right-hander Hector Rondon, the sixth Cubs pitcher, pitched a perfect ninth inning for his 17th save.

The Giants trimmed a three-run deficit to 5-4 with two outs in the sixth on Crawford's two-run homer to left off right-hander Tommy Hunter, making his second Cubs appearance.

Crawford's 19th home run of the season also drove in right fielder Hunter Pence.

Right fielder Jorge Soler gave the Cubs a 2-0 lead with a first-inning, two-out single to left. Soler's hit -- on a 3-1 Heston pitch -- drove in Schwarber and first baseman Anthony Rizzo.

Schwarber made it 5-0 in the second with his first career Wrigley Field homer, a one-out shot to right-center on a 1-0 pitch off Heston that also brought home Hammel and second baseman Addison Russell.

"(Heston has) had a couple starts like this -- he's bounced back," said Giants manager Bruce Bochy. "He's a tough kid. That first inning is so critical and you try to get settled in, but we could tell he was out of sync there and sometimes, it's hard to find it. When he walked a couple of guys there and hit a batter that set up the first rally, that's unlike him."

Schwarber has six home runs and 18 RBIs in 25 games since coming up June 16 for the first of two stints with the Cubs.

The Giants trimmed the lead to 5-2 with Belt's 14th homer of the year in the fourth. Belt's two-out blast came on a 2-2 pitch and brought home Pence.

Hammel allowed two runs on three hits, walked two and struck out four in his four-plus-inning outing.

Heston gave up five runs on five hits, walked two and struck out one in four innings.

"I actually felt really good in the bullpen," Heston said. "Just the first couple of innings I just had trouble finding the strike zone with both the breaking ball and the fastball. Saw a lot of hitter's counts and that's going to make it tough on anybody."

NOTES: San Francisco RHP Mike Leake (9-6, 3.52 ERA) will miss Friday's scheduled start with a hamstring strain. RHP Ryan Vogelsong (7-7, 4.16 ERA), who was shifted to the bullpen in mid-July, will start in Leake's place and face Cubs LHP Jon Lester (6-8, 3.26 ERA). ... Thursday's game against the Cubs opened a stretch during which the Giants face nothing but teams with winning records until they visit Colorado on Sept. 3. The next 26 games (eight series) are against playoff-caliber teams. ... San Francisco came into the series leading the majors with a .295 average with batters in scoring position (.436 in their past seven games though Wednesday). ... Giants C Buster Posey was out of the starting lineup. He began the day second in the National League with a .332 average. ... Chicago 3B Kris Bryant has 61 RBIs, tops among NL rookies. Giants 3B Matt Duffy is second with 51.