Barry Zito delivered on one of the biggest stages yet during six up-and-down years in San Francisco.

Zito pitched the NL West-leading Giants past the Los Angeles Dodgers 4-0 on Sunday night to extend their division lead to 5 1/2 games, coming through in among the most important outings of his tenure on this side of the bay.

"This game's obviously bigger because not only is it against our rival in the West, it's the Dodgers, too," Zito said. "Being 5 1/2 up, we like where we're at. We certainly can't rest on our laurels. We have to fight every day, and that's something we've been doing all year."

Hunter Pence hit a two-run double in the first inning that held up and Buster Posey homered in San Francisco's fifth win in the last six meetings of the rivalry.

Marco Scutaro added a sacrifice fly to back an impressive performance by Zito (11-8), who drew loud cheers all night from the orange flag-waving sellout crowd of 41,517. Zito, left off the postseason roster for all three rounds during the team's 2010 World Series title run, also matched a season high for wins since joining the Giants on a $126 million, seven-year contract before the 2007 season. He also had 11 in '07.

Los Angeles (74-67) dropped 1 1/2 games back of St. Louis for the second NL wild card with a crucial four-game set against the Cardinals starting Thursday at Dodger Stadium. Atlanta leads the wild card by 5 1/2 games.

San Francisco earned its NL-leading 13th shutout and fourth against the Dodgers. The Giants swept Los Angeles at AT&T Park from June 25-27 by scores of 8-0, 2-0 and 3-0. Zito and five relievers combined on Sunday's five-hit shutout.

"It stings us because it's a two-game swing with our trying to catch these guys," manager Don Mattingly said. "It changes from 3 1/2 to 5 1/2, but we can't be worried about that. ... We haven't done enough to win games."

The Dodgers scratched reigning NL Cy Young Award winner Clayton Kershaw a few hours before first pitch because of an inflamed right hip that required a cortisone shot Saturday.

Kershaw wasn't happy with the decision by Mattingly, saying, "I would have loved to pitch today." The lefty (12-7) is now scheduled to start Tuesday at Arizona following an off day for the club Monday.

Posey's career-best 20th home run came in a 10-pitch at-bat with six fouls against Joe Blanton (1-4) to start the sixth.

"That's one of the best at-bats I've ever had off of me," Blanton said. "I threw him at least five put-away pitches, I thought, and he just kept fouling them off."

Posey — the 2010 NL Rookie of the Year who missed most of '11 with a serious leg and ankle injury — has two such 10-pitch at-bats for home runs in his career, both this season in which he should challenge for NL MVP honors.

"I can't think of a guy who's more valuable for a club than Buster is for us," manager Bruce Bochy said.

Blanton (1-4) was told Saturday there was a "50-50 chance" he might be called upon to start in Kershaw's place. He needed 36 pitches to get out of the first, giving up as many runs that inning as Kershaw did total in any of his four starts facing San Francisco this year.

Angel Pagan matched his career high with an 11th triple, leaving him one shy of the San Francisco record. Hall of Famer Willie Mays hit 12 in 1960 and Steve Finley did so in 2006.

The Giants have all the momentum in the West — and hope to leave little to chance in the season's final week, when they end the year with three games at Dodger Stadium from Oct. 1-3.

"It would definitely be a little easier for us than to give them any chance of hope that they may have," reliever Sergio Romo said. "They've got to be able feed off it. For us to go in there and not have to worry about it, that would be huge for us."

Los Angeles is searching for timely hits after going 1 for 23 (.043) with runners in scoring position in the series.

Pitching to chants of "Zito! Zito!", the left-hander worked into the seventh inning and gave the Giants only their second home series win since being swept at home by the Dodgers on July 29.

Zito allowed four hits, struck out four and walked three in 6 1-3 scoreless innings to win his third straight decision. But it was just his third victory over the last seven starts and followed a pair of no-decisions by the 2002 AL Cy Young Award winner.

Zito left to a standing ovation when he gave way to Santiago Casilla with one out in the seventh and runners on first and second following a walk to A.J. Ellis. The pitcher raised his glove to acknowledge the fans.

"The atmosphere tonight was probably more electric than I've ever seen in one of my games," Zito said.

Casilla entered and retired pinch-hitter Bobby Abreu on a called third strike, then walked Mark Ellis before Shane Victorino left the bases loaded with a flyball to left.

Zito allowed a leadoff double to Mark Ellis to start the game but escaped unscathed — a big deal for a guy who is 10-3 this season when pitching a scoreless first inning and 1-5 when allowing a run.

Then, he got a quick lead in the bottom half.

Pagan singled then Pablo Sandoval walked one out later. A day after facing four total pitches in a 1-for-4 performance, Sandoval drew seven-pitch free pass. Posey struck out looking before Pence's double.

San Francisco has won Zito's last seven outings since an Aug. 2 loss to the New York Mets.

Notes: Zito is 10-1 this year when receiving four or more runs of support. ... San Francisco faces the division the rest of the way. ... Ryan Vogelsong (12-7) pitches for the Giants on Monday at Colorado trying to improve on his 5-0 mark with a 1.80 ERA in six previous starts vs. the Rockies. ... Los Angeles INF Adam Kennedy expects to miss the rest of the season with a strained right groin he hurt Friday night. "It's pretty bad," he said.