SAN FRANCISCO -- The more you win, Pittsburgh Pirates star Andrew McCutchen says, the more you win.

Surely, winning is getting contagious among the Pirates these days.

McCutchen capped a six-run fifth inning with a two-run home run Wednesday afternoon, sending the Pirates to a 6-5, come-from-behind victory over the San Francisco Giants and a sweep of the three-game series.

A second sweep in San Francisco in the last two seasons allowed the Pirates (62-56) to move within three games of the Giants (66-54) in the National League wild-card race.

The Pirates trailed the Giants by 12 1/2 games on June 23.

"That's the way the game goes," McCutchen said of the club's five wins in six games on their California swing through Los Angeles and San Francisco. "When you're playing well, good things happen. Regardless of what the score is, you feel you have a chance of winning.

"That's the way we're playing."

Giants starter Matt Cain had allowed only one baserunner -- a single by opposing pitcher Ivan Nova -- through four innings and held a 4-0 lead before imploding in the fifth.

Cain hit David Freese with a pitch to open the fifth, then issued consecutive walks to John Jaso, Jordy Mercer and Eric Fryer to force in a run.

Pinch hitter Matt Joyce followed with a two-run single to get Pittsburgh within 4-3, and the Pirates tied the game on Josh Harrison's sacrifice fly, the inning's first out.

After Cain got Starling Marte to fly to left field, McCutchen jumped on the first pitch he saw and crushed his 17th homer of the season, putting the Pirates ahead for good at 6-4.

"Look for something to hit," McCutchen said of his approach to the key at-bat. "You've got to simplify the game. The game's hard enough already."

Giants manager Bruce Bochy second-guessed himself afterward for allowing Cain to face McCutchen.

"I waited a little too long there," he said. "I felt like he had enough to get through it. It didn't play out. That's a tough one."

Before McCutchen's big hit, Pirates manager Clint Hurdle played his wild card -- ace pinch hitter Joyce -- earlier than usual, and it paid off.

"He's your guy," Hurdle said of Joyce, who leads the NL in pinch-hit RBIs with 15. "You take that shot with the bases loaded."

Relievers Jeff Locke, Juan Nicasio (9-6), Felipe Rivero and Tony Watson combined to hold the Giants to one run over the final five innings as the Pirates recorded their fifth series sweep of the season.

For the second game in a row, Watson pitched an adventurous ninth, loading the bases on a walk and two hits with no outs while protecting a 6-4 lead.

He got Buster Posey to ground into a double play, scoring Eduardo Nunez, before inducing Brandon Crawford to fly meekly to center field to secure his seventh save.

"This last week or two have really been fun for us," Joyce said. "It's a make-or-break time of year. It's nice to see some guys turn the corner. It's exciting to be a part of it."

Pittsburgh finished the NL West portion of its schedule with a 22-11 record. It went 27-6 against Western clubs last season.

The Giants, who lost their fourth straight and fifth in six games to open a 10-game homestand, jumped on Nova for a pair of two-run innings in the first and third.

Crawford did most of the damage, doubling home the second run of the first inning before smacking a two-run triple in the third.

Angel Pagan and Posey scored two runs apiece in the early uprisings.

Cain (4-8), who was pulled immediately following McCutchen's homer, allowed only three hits in his 4 2/3 innings. however, he was hurt by three walks and a hit batsman, all of which occurred in the fateful fifth.

Cain threw 37 of his 89 pitches in the fifth inning.

"When the guys have given us a lead with everything going on, we didn't need that to happen," he said. "That's a bad job by me."

The Pirates finished with just seven hits. Jaso and Harrison had doubles.

Nova allowed four runs on six hits in four innings. He struck out five in his third start as a Pirate, all of which Pittsburgh has won.

Posey and Pagan had three hits apiece, and Crawford added two of the Giants' 10 hits.

The Giants, who fell to 9-21 since the All-Star break, were swept for the fifth time this season.

NOTES: Three of the Pirates' five series sweeps have come on the road. ... OF Matt Joyce's 15 pinch-hit RBIs are one shy of the Pirates' record, set by 1B Willie Stargell in 1982. ... Eight of the Giants' 21 losses in the second half of the season have come by one run. ... San Francisco manager Bruce Bochy disclosed before the game that he is considering skipping RHP Matt Cain's turn in the rotation next week so that the Giants can align their top three pitchers -- LHP Madison Bumgarner, RHP Johnny Cueto and LHP Matt Moore -- to face the Los Angeles Dodgers in the series that begins on Tuesday. The Giants have a day off Monday, affording Bochy the luxury of potentially adjusting his rotation. ... Pirates RHP Tyler Glasnow (sore right shoulder) reported no discomfort after throwing a bullpen session Tuesday. He already is eligible to be reinstated from the disabled list. A decision whether to send him to the minors for an injury-rehab start has not been made.