"You expect to close that game down and we didn't do it, we gave in to it," D-backs manager Kirk Gibson said.
Buster Posey struck the winning blow in the ninth when he launched a two-out home run to left off Addison Reed. Miguel Montero hit a solo shot in the bottom of the inning to close the gap to a run before Sergio Romo locked down the save.
It was not the home debut Reed was looking for. The right-hander was acquired from the White Sox in exchange for third-base prospect Matt Davidson, and the D-backs are counting on him to bring stability to the closer's role.
Reed allowed a one-out single to Brandon Belt and struck out Pablo Sandoval before falling behind Posey, 1-0.
Reed tried to come in with a pitch but didn't get it where he wanted to, and Posey hit it deep into the bleachers.
"Terrible pitch, good swing," Reed said. "It was right over the middle of the plate; anybody is going to hit that a long way. I haven't gone back and looked at it, but missed my spot and he made me pay for it."
Said Posey, "I'd never faced [Reed] before, not even in Spring Training or anything. I was really just looking for a pitch, trying to get a pitch I could handle."
The game started much better than it ended for the D-backs as they gave the sellout crowd something to cheer about while they built a 7-3 lead through six innings.
At that point, all looked rosy for the D-backs as starter Brandon McCarthy followed up a stellar spring with a quality outing while offseason acquisition Mark Trumbo had three hits and outfielder Gerardo Parra showed he could hit lefties, driving home a pair of runs off Madison Bumgarner.
Then came the top of the seventh and one pitch that seemed to turn the tide.
McCarthy got two outs in the inning, but it was the third one that would prove elusive for him and a pair of relievers.
With two outs and a runner on first, McCarthy thought he had pinch-hitter Ehire Adrianza struck out on a 2-2 pitch, but it was ruled a ball. Adrianza then doubled home a run on the next pitch, spelling the end of the night for McCarthy.
"[Ticked] off," McCarthy said of his feeling after the game. "There was a call there in the seventh that changed the entire outlook of the game. Obviously we all have to execute after that point, but that's a juncture, that's a pitch that you're not rewarded for, so that kind of leaves a bitter taste. The next pitch was a 3-2 sinker down and away, and looking at it, it was right where I want it, and that's a credit to the young kid to stay on that and be able to hook that ball, but the pitch before ... no, I did have him struck out, it just didn't get called."
Left-hander Oliver Perez then allowed three straight hits and Brad Ziegler allowed a single and a walk with the bases loaded as the Giants scored four runs to tie the game.
"You know the Giants are resilient, they're going to come back on you, but we didn't have enough runs tonight," Gibson said. "We got to take this; it stings a bit, but we're going to have to regroup and come back tomorrow."
Last season, the D-backs bullpen blew 29 saves, which tied for most in the Majors, and the team made improving the relief corps a priority during the offseason, trading for Reed while signing Perez to a two-year free-agent deal.
"Absolutely," Reed said when asked if it was a disappointing way to start. "Especially when our offense kept us in the game the whole entire time and it got to me and things didn't work out as planned. Get ready and get after them tomorrow."
The D-backs are now 0-3 on the season, having dropped a pair of games to the Dodgers on March 22-23 in Sydney, Australia.