PITTSBURGH -- Brandon Belt is caught up in it, right along with his San Francisco Giants teammates.

It has been a mostly miserable season, but suddenly the wins are pouring in.

Belt drove in three runs Sunday as the Giants erased a two-run deficit to gain a weekend sweep with a 5-3 win over the Pittsburgh Pirates at PNC Park.

Belt was 3-for-4 with his 16th homer and an RBI single as San Francisco (33-51) won its sixth in a row and got its second straight series sweep. The Giants' last sweep in Pittsburgh was April 29-May 1, 2005.

"I think there's a confidence and expectation to win now, whereas before I don't think we had that," Belt said. "It actually might have been the opposite. Not sure what triggered it, but this is the way we feel we should've been playing the whole time. This is how our team should be playing."

San Francisco starter Jeff Samardzija (4-9) allowed two runs, both on Jordy Mercer's fourth-inning homer, and six hits in six innings with five strikeouts and no walks.

In his past 11 starts, Samardzija has walked just four while striking out 87. He has held opponents to three runs or fewer in nine of his past 10 starts.

"We're going out there and I don't think we're looking it as a whole right now," Samardzija said. "I think we're looking at it as a game-by-game moving forward. I just think we need to go out every day and set a tone with the way we start. ... You can just do yourself a favor and try to go out and have fun."

Sam Dyson pitched a perfect ninth for his second save.

Tony Watson (4-2) took the loss for Pittsburgh (37-45). He gave up a run and three hits in the seventh.

Trevor Williams, making his first career start against the Giants, got his fifth straight no-decision. In six-plus innings, he gave up two runs and three hits with three strikeouts and two walks.

Pittsburgh had a runner in each of the first three innings before taking a 2-0 lead in the fourth. Mercer's eighth homer, into the left-field bleachers, followed Gregory Polanco's one-out double. It was the 17th homer off Samardzija this season.

The Giants chased Williams in the seventh after Hunter Pence led off with a walk and went to third on Buster Posey's double to the left-field corner.

"I pulled a few pitches to Pence," Williams said. "Posey, I made a good pitch, really similar to the strikeout that he had his first at-bat. He's a good hitter."

Getting the two runners seemed to ignite San Francisco.

"We couldn't get anything generated, then finally the leadoff walk seemed to get things going," Giants manager Bruce Bochy said.

And left Williams feeling bad about what he handed over to Watson.

"Second and third, no outs, is not a fun situation to come in as a reliever," Williams said.

Belt singled on Watson's first pitch, driving in Pence to make it 2-1, and Brandon Crawford followed with an RBI single to tie it.

Belt scored on pinch-hitter Kelby Tomlinson's sacrifice fly for a 3-2 Giants lead, drawing boos toward Watson from the home crowd.

"Your other option is to give Trevor the opportunity to get through it, and it he doesn't have a lot of experience pitching through a jam in the seventh," Pirates manager Clint Hurdle said. "With those left-handers coming up, something Tony's been able to (handle), he's been in a good place. We weren't able to get it done."

In the eighth, Belt sent a two-out, two-run homer into the right-field stands off Edgar Santana to boost the lead to 5-2.

John Jaso hit his seventh homer of the season to right -- and just his second career home run off a left-hander, Steve Okert -- to pull the Pirates within 5-3 in the eighth.

NOTES: Pittsburgh OF Starling Marte, serving an 80-game suspension for a violation involving performance-enhancers, was 0-for-3 with a strikeout and reached on an error in five innings in his first rehab game with High-A Bradenton. Marte, who is eligible to return July 18, will play left field, manager Clint Hurdle said. ... CF Andrew McCutchen, 2B Josh Harrison and 3B David Freese, all Pittsburgh regulars, were out of the starting lineup for rest purposes. ... Giants 3B Ryder Jones, who was hit by a pitch on the left wrist Saturday, did not play but is not expected to miss many, if any, more games. ... San Francisco 3B Christian Arroyo will miss at least two months with a broken left middle finger. He was hit by a pitch in first rehab game with Triple-A Sacramento on Saturday while coming back from an injury to the same hand.