Cubs 6, Giants 4: Alfonso Soriano and Anthony Rizzo hit home runs and Chris Volstad turned in a second straight solid start as Chicago beat visiting San Francisco in the opener of a three-game series.

It was the third loss in 11 games for the Giants, who maintained a 4 1/2-game lead over the Los Angeles Dodgers in the National League West.

Volstad (2-9) won his second straight start after 24 consecutive outings without a victory. He allowed two runs on five hits in 5 2/3 innings.

The Cubs jumped on left-hander Madison Bumgarner early as Soriano started the scoring with an RBI single in the first and added a two-run homer - his 24th - in the four-run third. Welington Castillo added an RBI double and scored on Buster Posey's error to cap the big inning and make it 5-0.

Rizzo added his 10th home run in the fifth, a solo shot that bounced back onto the field and initially was ruled in play. Rizzo wound up at third, but the play was overturned and ruled a home run after video review.

San Francisco rallied for two in the eighth against Shawn Camp to get within 6-4, but Carlos Marmol struck out two in a perfect ninth for his 17th save.

Posey went 3-for-3 with two RBIs for the Giants.

Bumgarner (14-9) allowed five runs (four earned) on six hits over four innings to lose his second straight start.

GAME NOTEBOOK: The Cubs have won two straight after losing 22 of their previous 28 games. ... The Giants lost for only the sixth time in their last 23 road games. ... Giants INF Joaquin Arias extended his hitting streak to nine games, matching a career high. ... Cubs SS Starlin Castro extended his hitting streak to seven games.