CHICAGO — Alfonso Soriano hit a two-run homer that landed on Waveland Avenue and Anthony Rizzo added a solo shot, helping Chris Volstad and the Chicago Cubs beat the weary San Francisco Giants 6-4 on Friday.

Soriano also had a two-out RBI single in the first as Chicago earned its first win of the season against the NL West leaders in five tries. The Cubs were swept in a four-game series at San Francisco from June 1-4.

Buster Posey went 3 for 3 with two RBIs for San Francisco, which had won a season-high six consecutive road games. Pablo Sandoval had a sacrifice fly and Hunter Pence drove in a run with a groundout.

The Giants rallied for an 8-4 victory in Houston on Thursday night, then arrived in Chicago around 3 a.m. They mostly stayed in the visitors' clubhouse while the Cubs took batting practice, but they looked listless for much of the muggy afternoon at Wrigley Field.

It was still quite a month for San Francisco, which went 18-11 in August to increase its division lead over the Los Angeles Dodgers. It was the most victories for the Giants in a single month since they had 18 in September 2010, and they went on to win the World Series that year.

Volstad (2-9) allowed two runs and five hits in 5 2-3 innings for his second consecutive victory after a 24-start winless streak. The 6-foot-8 right-hander pitched into the seventh against Colorado on Sunday for his first win since July 10, 2011, for the Marlins against Houston.

Madison Bumgarner (14-9) won three of his first four August starts for San Francisco, but ended up dropping his last two after another ineffective outing against Chicago.

The left-hander retired the first two batters in the third before the Cubs strung together four consecutive hits to build a 5-0 lead. Rizzo got it started with a single to center and Soriano followed with a drive to left on an 0-2 pitch for his 24th homer. Starlin Castro then singled and scored all the way from first on Welington Castillo's double to the gap in left-center.

Castillo moved to third on the throw home and headed for the plate when the ball got away from Posey. Bumgarner's return throw was high and went off the All-Star catcher's glove, allowing Castillo to score easily.

Rizzo added his 10th homer in the fifth, a drive to left-center that skipped off the basket and caromed back onto the field. The big first baseman hustled into third, but the drive was eventually changed to a home run after a short replay review.

Despite the short turnaround from the series in Houston, the Giants almost rallied against the Cubs' bullpen. Brandon Belt flied out to the warning track with two runners on to end the sixth, then flied out to center with a runner on for the final out in the eighth.

Carlos Marmol got the final three outs for his 17th save in 19 chances. The right-hander has converted 16 straight save opportunities.

The Cubs closed the month with a pair of impressive wins, but still finished August with an 8-21 record. They scored three times in the ninth on Thursday for 12-11 victory over Milwaukee on Soriano's game-ending single.

NOTES: Giants manager Bruce Bochy said Posey will get a day off this weekend. San Francisco plays three consecutive afternoon games in Chicago before returning home for another afternoon start against Arizona. ... The Cubs recalled reliever Jeff Beliveau from Triple-A Iowa and sent fellow LHP Brooks Raley to the minors. They also announced they will recall OFs Tony Campana and Dave Sappelt and INF Adrian Cardenas when major league rosters expand on Saturday. Raley's season is over, and LHP Chris Rusin will be recalled on Tuesday to start at Washington. RHP Miguel Socolovich, who was claimed off waivers from Baltimore on Aug. 23, will be added to the active roster on Sunday. Backup C Anthony Recker, who was acquired in a trade with Oakland, is expected to join the team on Tuesday. ... RHP Tim Lincecum (7-14, 5.30 ERA) starts for San Francisco on Saturday against RHP Justin Germano (2-4, 5.91 ERA). Lincecum is 2-0 with a 1.59 ERA in four career starts at Wrigley Field.

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Jay Cohen can be reached at http://www.twitter.com/jcohenap