SAN FRANCISCO -- The San Francisco Giants were impressed when Christian Arroyo, in his first major-league at-bat, hit a ground ball to the right side that advanced a baserunner to third.

They hadn't seen anything yet.

The 21-year-old homered for the second time in three games Friday night, a tiebreaking solo shot on reliever Ryan Buchter's second pitch of the eighth inning, lifting the Giants to a 4-3 victory over the San Diego Padres in the opener of a three-game series.

Right-hander Jeff Samardzija and two relievers combined on a five-hitter, delivering the Giants their third one-run win in five games on their current homestand.

"A couple of us heard he was 21. We laughed at it,” Samardzija said of Arroyo. "He's a smart player. That first at-bat (Monday) says it all."

The Giants' win came despite Padres third baseman Ryan Schimpf's third consecutive game with a home run, a two-run drive in a three-run fifth inning off Samardzija.

"I'm not thinking about it to be honest with you,” Schimpf said of his streak. "Really, as cliche as it may be, it's just about trying to stay prepared, put your work in, and try to put together a good at-bat. Try to put your best swing on it, and whatever happens after that, it's out of your control.”

Arroyo, the Giants' first-round pick in 2013, picked on a 1-0 fastball from the left-handed Buchter and sent it into the left field bleachers for his second home run of the fairy-tale week.

"I was not trying to hit a home run," he insisted. "Just get on base and keep the line moving. But I got a good pitch and got the barrel on it and I was fortunate that it went out."

The homer was the first allowed by Buchter (1-1) in 10 appearances this season.

Right-hander Derek Law (3-0), who stranded Erick Aybar at third base in the top of the eighth inning to retain a 3-3 tie, got the win.

Giants closer Mark Melancon pitched a 1-2-3 ninth for his fifth save.

"He's been a huge shot in the arm," Giants manager Bruce Bochy said of Arroyo. "He's helped us win some games here."

Neither starting pitcher received a decision.

Samardzija left for a pinch hitter in the bottom of the seventh, having allowed three runs (two earned) and four hits in seven innings. He walked one and struck out five.

Padres right-hander Luis Perdomo was pulled after six innings, during which he allowed three runs (one earned) and six hits. He struck out five and did not walk a batter.

"The most important thing is that I felt good," said Perdomo, who missed two starts earlier this month with shoulder tendinitis. "I was relaxed (and) felt like I had my pitches working."

Brandon Belt had three hits, including a double, and Joe Panik collected two singles for the Giants, who outhit the Padres 8-5.

Cory Spangenberg had a single and a double for the Padres.

NOTES: The Padres' record for home runs in consecutive games is six, set by 3B Graig Nettles in 1984. ... Padres 2B Yangervis Solarte was hit twice in the game by Giants starter RHP Jeff Samardzija, raising his season total to four. Giants pitchers have accounted for three of the four HBPs. ... The Giants' last four wins have all been by one run. ... The Giants had scored just four first-inning runs before getting two on Friday. ... 1B/LF Michael Morse's sacrifice fly was San Francisco's major-league-leading 11th of the season.