A potential pitchers' duel gone awry put this matchup in the hands of the bullpens, and Detroit prevailed, with A's relievers Dan Otero and Fernando Abad combining to allow two crucial runs.
Otero erased Oakland's one-run lead by allowing a one-out solo homer to Torii Hunter in the seventh, and Abad walked two in a row in the eighth to set up the Tigers' go-ahead run.
But perhaps the biggest mistake made by his Abad was his inattention of pinch-runner Davis. With one out, the former A's speedster bolted from second to third -- after making it to second on Derek Norris' passed ball -- on Norris' throw back to Abad, both of them seemingly oblivious of Davis.
Davis scored the go-ahead run on Austin Jackson's ensuing ground-ball forceout off righty Luke Gregerson, leading to the A's fifth loss in their last six games.
Neither Abad nor Norris hung around to answer reporters' questions after the game -- and their manager wasn't ready to fully defend them, having not seen the replay yet.
"You always have to be ready for something like that with a guy that's that fast," said Bob Melvin. "I haven't seen the video -- I don't know, once Abad had the ball, if he threw right away, if he had a chance or not. So that's something I'm gonna have to look at.
"[Davis] timed it perfectly. As soon as [Norris] let go of the ball, he took off."
"That's my specialty," said Davis. "I thought it was possible after the first throw back from the catcher. I took the circumstances into consideration. No one was expecting it. It does take a little courage to do that at that time in the game. You have to be safe. If you get thrown out, it's heavy."
It was Coco Crisp who nearly stole third base in the first inning, after leading off the frame with a double against righty Max Scherzer. But he was caught stealing -- a call confirmed on replay -- for just the second time in 10 attempts this season, letting Scherzer off the hook.
Melvin says he has "as much faith in Coco as a base stealer as anybody I've been around," though this turned into a contest where every little miscue was magnified.
Starter Sonny Gray shouldered some of the blame for this one, saying he needed to be better, particularly on a night when Scherzer wasn't at his best, either.
Gray tied a season high by allowing four runs but completed six innings for the 11th time in as many tries this season.
His catcher, John Jaso, back in the starting lineup for the first time since Thursday, worked Scherzer for 22 pitches all on his own, hammering one of them into the right-field stands for a go-ahead two-run homer in the fourth.
For some time it seemed that would prove the difference in this one. Instead, the same club that has eliminated the A's from the postseason after just one round in two consecutive years mounted a comeback and evened this four-game series at a game apiece.
Both times, Justin Verlander has beaten them in the decisive Game 5, so there were few sad faces in the A's clubhouse when it became known he would not be pitching in Oakland this week.
Still, the A's were left to battle with Scherzer, who held them to just three runs over nine innings in the playoffs last year. This time around they took control, forcing him to throw 85 pitches through the first four innings and 107 overall through six frames.
Jaso saw 22 of those pitches, drawing a nine-pitch walk in the first and getting hit by the 11th pitch of his third-inning at-bat. He only needed to see one in the fourth before drilling a homer to right field to score Callaspo, who reached on a base hit and advanced to second on a balk that plated Josh Reddick.
Gray was out of sorts from the start, boarding four consecutive batters with just one out in the first inning, allowing a double to Hunter and an ensuing RBI base hit to Miguel Cabrera, who would also score on another double from J.D. Martinez.
Cabrera proved trouble again in the third, tagging Gray for a solo shot to left field. The Tigers would add on in the next inning on Alex Avila's RBI double.
"I felt really good. That's why it's a little bit frustrating," said Gray. "You felt like you have good stuff, and then you just get hit around a little bit."
"It just seemed like his two-seamer wasn't moving like it normally does," said Jaso, "and that's where he ran into trouble."
Gray was left with a no-decision, and the A's bullpen is now responsible for 10 of the club's 21 losses.