PHOENIX — Matt Cain pitched seven brilliant innings Sunday, and the San Francisco Giants positioned him for a victory when Pablo Sandoval hit a tiebreaking, three-run home run in the eighth.

But before anyone could think to rub Cain's ears for good luck, his decision — and a three-game sweep over the Arizona Diamondbacks — tumbled away in the ninth. Brian Wilson gave up a tying, two-run homer to Justin Upton, and Conor Jackson hit a bases-loaded single in the 12th off Justin Miller as the Giants lost 5-4 at Chase Field.

Wilson blew his first save in four chances, allowing hits to three of the first four batters he faced. Upton smoked a low line drive that struck the padding at the top of the left-field fence and skipped over.

"You get a game you think is in the bag and it slips away," Wilson said. "It's disheartening.

If the universe favors equilibrium, it owes Cain a lot of the good stuff.

Last season, he was one of 20 major league pitchers to post at least 20 quality starts (minimum of six innings, maximum of three earned runs). Nineteen of those pitchers had a winning record. Cain was 8-14.

He held the Diamondbacks to four hits in seven innings, but his spot led off the eighth and Manager Bruce Bochy sent up pinch hitter Fred Lewis. He doubled, and Edgar Renteria walked to set the table for Sandoval, who tied a career high with four hits.

Sandoval, who entered with one RBI and was 3 for 19 with runners in scoring position, hit the first home run by a Giants corner infielder in 130 plate appearances this season. The Giants were the only team in the major leagues that didn't have a home run from its corner infielders.

Cain stood to improve to 3-0. The last time he had three more wins than losses was in September 2006, when he was 13-10.

Catcher Bengie Molina didn't curse the fates after Wilson blew Cain's decision.

"I hope we don't believe in luck," Molina said. "I hope we go out and make pitches when we have to and let that decide our fate. I believe in the hard work you put in on the field."

While Sandoval provided the big hit, Bochy bemoaned the club's 2-for-15 performance with runners in scoring position. Emmanuel Burriss got picked off in the ninth. Sandoval and Nate Schierholtz hit leadoff doubles in the sixth and seventh but ran themselves into outs. Juan Uribe led off the 12th with a double; the next three hitters failed to advance him.

"Our situational hitting was off today, and our baserunning," Bochy said. "We had chances to put the game away. We didn't execute."

The Giants are operating with an 11-man staff and Bochy seemed determined to stay away from right-hander Merkin Valdez, so Brandon Medders went out for a third inning in the 12th. He issued three walks, one intentional, and Miller fell behind Jackson with no place to put him.

"We're going to have to use Valdie, we know it," said Bochy, adding the club could make a roster move to add a pitcher before it starts a homestand with the Los Angeles Dodgers today.

At least the Giants have to feel good about Cain, who pitched efficiently despite four walks. He threw 103 pitches and has a 2.08 ERA through four impressive starts. He used his breaking ball to throw first-pitch strikes to Arizona's aggressive, fastball-loving lineup.

"The biggest thing is to get ahead and really get them aggressive," Cain said. "We'll go against some good starters in this division, and it comes down to you being able to stay in longer."

Wilson was pitching a second consecutive day and threw two innings Wednesday, but he didn't make excuses.

"Fatigue is not something that enters my body," he said. "Just one of those days I couldn't get my fastball in the zone and the slider was breaking out."