By dropping a 5-2 game to the Giants on Wednesday evening, after allowing five runs total in their previous six games, the A's have now lost 13 of their last 15 and 18 of 25 overall at AT&T Park.
Back at home, they've won 11 of their last 13 against them.
"I think when you have two teams that play competitive ball against each other, a lot of times home field will come into play," said manager Bob Melvin. "We do have a good road record this year, but I'm sure they got a boost by their crowd tonight as well."
"Playing here, they've got a great atmosphere here just like we do across the bay," added Stephen Vogt. "These two fan bases are outstanding. It's a fun place to play. Whether it's a home-field advantage thing, I don't know, but we had our opportunities tonight. Hopefully tomorrow we can come through."
Thursday's matinee marks the end of this year's Battle of the Bay, with the A's carrying a 2-1 edge entering play.
Right-hander Jason Hammel called the National League home for the first three months of the season, before joining Jeff Samardzija on a one-way flight to Oakland following their Friday trade from the Cubs. But this was his first taste of the Bay Bridge rivalry.
He lasted five innings in his A's debut, struggling with command at times but limiting the damage to two earned runs (three total) on six hits, including a solo shot to Hunter Pence in the fourth. He struck out just as many as he walked, issuing a season-high three free passes.
Melvin initially planned to leave him in for the sixth if not for a left thumb spasm that delayed the fifth -- a cramp that had the appearance of a dislocated finger.
"That's what I thought, too, but it was a cramp," said Melvin. "He was having a tough time getting it loose. I was thinking about letting him go back out there, but thought a little differently after that."
"I couldn't get it to straighten out, so I had to stop and figure out what was going on," said Hammel. "I've never had that happen to me before. My thumb cramped up and then my hand completely just went straight down, so it was stuck for a little bit. But we'll be all right."
Hammel, who finished his night at 99 pitches, fell to 0-3 in 10 appearances, including nine starts, against the Giants in his career, despite owning a 3.07 ERA vs. them.
"I thought he threw the ball well," said Vogt. "I think they made him throw a lot of pitches. They did a good job against him. I don't think it was necessarily anything wrong with him. They had a good approach. They've seen him a lot. So he's new to us but not new to them, so they probably had a pretty good game plan against him."
"Just inconsistent command," Hammel said. "That's just not me, walking guys, so I got myself into trouble putting guys on base, and also putting guys away -- two-strike pitches that were too hittable.
"My slider is usually my go-to pitch, and just wasn't commanding it tonight, and the fastball was up. I got myself into trouble with those deep counts and drove the pitch count up."
The righty walked Gregor Blanco on four pitches with one out in the second, setting up a base hit from Joe Panik that Yoenis Cespedes bobbled long enough in left field for Blanco to score.
Hammel then labored in a 37-pitch third inning but limited the damage to one run, and the A's got it back quickly in the fourth on a leadoff homer from Vogt, who finished with two hits in the same ballpark he frequented as a teenager to extend his hitting streak to a career-high nine games.
His teammates managed just three other hits, though, and Jed Lowrie's RBI single off Matt Cain in the sixth brought in their final run.
San Francisco added on in the bottom of the inning with two runs off lefty Eric O'Flaherty, and the A's exited with just their seventh loss in their last 24 games.
Hammel will have plenty more chances at a win in green and gold.
"He's got good stuff," said Melvin. "Probably not exactly what he would have hoped for, but we didn't do him any favors in the field, really, or swinging the bats."