Phoenix, AZ (SportsNetwork.com) - The reigning World Series champion San Francisco Giants got off to a strong start behind their ace, then they had to hold on in the latter innings before grabbing a 5-4 win over the Arizona Diamondbacks.

Madison Bumgarner (1-0), the 2014 NLCS MVP and World Series MVP, allowed one run on six hits over seven innings.

Bumgarner exited the contest and Arizona proceeded to load the bases in the eighth inning off relievers Javier Lopez and Jean Machi. Sergio Romo then entered and gave up a bases-clearing double to Jake Lamb that made it a one- run game with one out.

Romo struck out Tuffy Gosewisch before putting pinch-hitter Cliff Pennington on with a walk. Jeremy Affeldt was called upon to work out of trouble and he got David Peralta to go down on strikes to end the inning.

Santiago Casilla came on in the ninth and secured the win with a 1-2-3 inning, striking out Paul Goldschmidt to end the game and spoiling Arizona manager Chip Hale's debut.

The first three batters of San Francisco's lineup -- Nori Aoki, Joe Panik and Angel Pagan -- combined to go 8-for-14 with four runs scored. Pagan and Brandon Crawford each knocked in two runs, while Buster Posey added an RBI in the win.

"When we're clicking, it's just going to be a smooth transition from one guy to the next," said Panik. "When we're going, it's going to be fun to watch."

Mark Trumbo's RBI triple accounted for Arizona's other run. Josh Collmenter (0-1), making his first career start on Opening Day, was charged with the loss after surrendering five runs on 10 hits over 4 2/3 innings.

"We have to battle for these next two games," Hale said. "They showed a lot of fight. It felt great to come back."

The Giants threatened in the first inning, as Panik singled before Pagan doubled down the right-field line. Collmenter worked out of the early jam, however, striking out Posey and getting Brandon Belt to fly out to end the inning.

The top of San Francisco's order came back around in the third and broke open the scoring with a run on three hits. The Giants may have been able to get more, but Aoki, who singled with one out, was caught in a rundown after third base coach Roberto Kelly put up a late stop sign on Panik's double to right center. Pagan picked up his newly-appointed coach, though, driving in Panik with a single to center.

Trumbo tied the game in the home third with his triple off the overhang in center field that plated Ender Inciarte.

Aoki set the table in the fifth for the visitors, leading off with a lined single to right. Panik collected his third hit with a single to left, then Pagan sent a double into right that scored Aoki. Posey chased Panik home with a sacrifice fly to left in the next at-bat for a 3-1 lead.

Andrew Chafin relieved Collmenter two batters later following a walk to Casey McGehee, and he immediately served up a two-run double to Crawford.

Game Notes

Former Diamondback and 2015 Hall of Fame inductee Randy Johnson threw out the ceremonial first pitch ... The Giants have won 10 of their last 12 against Arizona ... Bumgarner improved to 7-4 career against the D-backs ... Goldschmidt, who missed the final two months of last season with a broken left hand, went 0-for-4 with a run scored ... Pagan reached base safely in four of his five at-bats ... San Francisco opened the season on the road for the sixth consecutive year.