The defeat, which featured Chris Davis' 42nd homer of the season, dropped the third-place O's to 64-52 and squandered a quality start from lefty Wei-Yin Chen in the process.
"Not quite enough," Davis said of his 466-foot, eighth-inning bomb, which was the longest homer of his career. "Chen pitched a really good game. Obviously ran into trouble a little bit late, but I think that's the way the series has been so far. Every little run helps.
"I know their record doesn't say it, but they're a good team. Any time you come in here, you're going to have a battle on your hands. That's the reason they've won the World Series the last few years. They know how to win."
Davis' long ball off Santiago Casilla, which extended Davis' Major League lead and gave him 109 RBIs on the season, pulled the O's within a run. They threatened to close that gap one out later with Nick Markakis -- batting fifth for the first time this season -- collecting his third hit of the afternoon and J.J. Hardy following with an infield single to shortstop Joaquin Arias. But Ryan Flaherty went down looking at a called third strike and Casilla got pinch-hitter Matt Wieters to strike out to quash the threat.
It was a familiar scene for the Orioles, whose only other run came on Adam Jones' sacrifice fly off Giants starter Chad Gaudin.
"We were fortunate we had a lot of borderline pitches with Gaudin go our way that [we] could have not even had those situations," said Orioles manager Buck Showalter, who watched as Gaudin stranded seven runners in his five frames.
"But you still want to cash in on them over the course of a game. I'm trying to be nice, but borderline pitches today we seemed to benefit from it a little bit. Just didn't cash the ticket when we had some opportunities. We got the game in some situations where we liked our chances; we just weren't able to get it done today compared to last night."
Gaudin issued a pair of one-out walks in the second inning, following Markakis' single, to load the bases, but the O's kept coming up empty. Taylor Teagarden struck out, and Chen, after jumping out to a 3-0 lead, bounced a full-count pitch back to the mound.
"It started with Chad," Giants manager Bruce Bochy said of his club's pitching prowess. "He logged some pitches there, but he competed so well out there to keep them to one run. Guillermo [Moscoso] had two solid innings. We needed it. He came through for us."
Jones and Markakis started the sixth inning with a pair of blooper singles off Moscoso, who got a pair of flyouts and struck out Teagarden to strand them there.
"Timing aspects are what I struggle with the most," said the rarely used Teagarden, who got the start in place of Wieters and went 0-for-3 with two strikeouts. "I'm facing guys I've never even seen before in my career, what pitches they have, get ready to hit at the same time. I'm just trying to do the best I can. I'm happy when I get in there. It is what it is. I try to battle."
The offensive shortcomings saddled Chen with his fifth loss of the season in an outing that started off in dominant fashion. On the heels of Chris Tillman's eight-inning gem, Chen needed just 52 pitches to get through the first five scoreless innings, and he retired 13 consecutive batters before Andres Torres' leadoff single in the sixth.
After a sacrifice bunt moved Torres to second, Marco Scutaro tied it up with a single into center field. Chen retired Arias on a fading liner for the second out but couldn't hold down Brandon Belt, who blasted a double that bounced over the center-field fence to put runners on second and third. Chen intentionally walked Buster Posey to get to Hunter Pence, who made him pay with a two-run double into the right-field corner that gave San Francisco its first lead of the series.
"Not at all," Chen said, through his interpreter, of being in the on-deck circle ready to hit at the top of the inning disrupting his rhythm. "I think the big inning in the sixth, that's all my fault because everything was high and I couldn't tell the strike zone."
Chen was lifted after six innings for pinch-hitter Brian Roberts and would have gone deeper into the 72-pitch outing had it not been his turn to hit. In Chen's last 38 starts, the Orioles have now scored three or fewer runs while he was in the game 25 times.
"He was good just about the entire outing," said Showalter, who got scoreless innings from Francisco Rodriguez and Darren O'Day after that. "Just when we get out to that 1-0, 2-0 lead like we had, would be nice to be able to expand it a little bit. But they are pitching well, too. I thought he deserved a little better fate, too, Gaudin. Their bullpen did a nice job. We had some good situations, just didn't cash the ticket."
The Orioles' lone run off Gaudin came after Nate McLouth's leadoff third-inning walk and a throwing error by first baseman Belt that allowed Davis to reach second, putting McLouth 90 feet away. Red-hot Jones drove him in to mark his seventh RBI in eight games.