SAN FRANCISCO -- San Francisco Giants ace Madison Bumgarner insisted he pitched better Thursday night than the scoreboard indicated.

Unfortunately for the left-hander, the box score indicated he is still winless in 2017.

Hunter Renfroe and Cory Spangenberg hit two-run homers off Bumgarner, propelling the San Diego Padres to a 5-2 victory in the opener of a four-game series.

Right-hander Jhoulys Chacin prevailed in a duel with Bumgarner for the second time in a week, limiting the Giants to two runs and four hits in six innings. He helped San Diego snap a three-game losing streak.

"When you pitch against (Bumgarner), you really have to make pitches and keep your team there, because he's never going to give up," Chacin said. "I felt really good tonight."

The Giants, who have lost 15 of their past 20 matchups with the Padres, had a three-game winning streak end.

Spangenberg broke a 2-2 tie in the seventh inning with a 415-foot home run to right-center field after Erick Aybar reached with a one-out single against Bumgarner. The homer was Spangenberg's eighth of the season.

"I've got to stop giving up homers," said Bumgarner, who surrendered a pair to the Padres for a second consecutive game. "I really felt good both times (against the Padres). That's what's frustrating."

The hit in the lefty-lefty matchup was the first of Spangenberg's career against Bumgarner after an 0-for-8 drought that included four strikeouts.

"I'm really impressed," Padres manager Andy Green said of Spangenberg's homer. "That's a long way out there. It'd take me two times hitting it to get it out that far."

Bumgarner (0-4) was pulled after the homer, having allowed four runs and six hits in 6 1/3 innings. He walked one and struck out five.

He was making his first home start since sustaining a separated shoulder in a dirt bike accident in Colorado in April.

"Solid outing," Giants manager Bruce Bochy said. "We've got to find a way to score a few more runs."

Renfroe's homer, his 17th, came in the second inning, in his first at-bat after being reinstated from the disabled list. He missed the Padres' previous seven games with a strained neck.

The rookie has hit two home runs in 10 career at-bats against Bumgarner.

Chacin (9-7) walked one and struck out four. He won for the fourth time in five career decisions at AT&T Park.

"Those first five innings from him were as good as he's been all year," Green said.

Bochy agreed.

"Chacin's been tough on us -- and not just this year," he said. "Probably next to (Clayton Kershaw), I don't know who's tougher on us."

Ahead 5-2, the Padres survived a bases-loaded Giants threat in the eighth with left-hander Brad Hand on the mound.

In a battle of All-Stars, Hand got Buster Posey to fly to right field with two outs and the potential tying runs on the bases.

Padres closer Brandon Maurer pitched a two-hit ninth inning for his 20th save, striking out pinch hitter Nick Hundley as the potential tying run to end the game.

Denard Span and Eduardo Nunez had RBI singles in the sixth inning to account for all of the Giants' scoring.

Spangenberg and Aybar had two hits apiece for the Padres, whose nine hits included four for extra bases.

Span, Nunez and Gorkys Hernandez collected two hits apiece for the Giants, who lost two of three to the Padres in San Diego last week.

NOTES: The Giants have given LHP Madison Bumgarner a total of 10 runs of offensive support in his six starts this season. ... Bumgarner allowed two home runs for a second consecutive game, the first time he has given up multiple homers in back-to-back games. ... With his ninth win, RHP Jhoulys Chacin equaled the total of the Padres' top winner last season, RHP Luis Perdomo. ... Padres RF Hunter Renfroe took the roster spot of CF Manuel Margot, who was placed on the three-day paternity list Thursday.