Giants 4, Cubs 3: Madison Bumgarner struck out 11 over eight-plus innings and host San Francisco staved off a ninth-inning Chicago rally to win the opener of the four-game series.

After the Cubs’ first two batters reached base in the ninth, Santiago Casilla replaced Bumgarner and promptly gave up a three-run homer to Alfonso Soriano.

The Cubs put two more runners on against Casilla before Javier Lopez came on to retire Steve Clevenger and David DeJesus for his first save.

Bumgarner (6-4) gave up two runs on eight hits with no walks over 8 1/3 innings to win for the first time in his last five starts as the Giants moved four games behind the first-place Dodgers.

Ryan Theriot went 3-for-3 and gave the Giants an early lead with his RBI double in the first off Paul Maholm (4-4), who allowed four runs over 5 1/3 innings.

Chicago, which was coming off a three-game sweep of San Diego, lost its eighth straight road game and fell to 6-18 away from Wrigley Field.

Leading 2-0, the Giants extended their lead in the third inning when Melky Cabrera tripled in a run and scored on Buster Posey’s single.

GAME NOTEBOOK: Bumgarner, who is still seeking his first career complete game, improved to 12-2 in his last 15 starts at AT&T Park. … Giants OF Angel Pagan extended his home hitting streak to a franchise-record 25 games with his single in the third inning. … Casilla was limping with an apparent leg injury when he left the field in the ninth inning. … Cubs reliever Carlos Marmol struck out two of the three batters he faced in the eighth inning.