On a toasty night at Camden Yards, the Orioles turned to their hottest player to get them out of the hole.
In a wild game before 39,129 fans, each team slugged four home runs and Baltimore got six of its 13 hits in a six-run seventh inning.
"To battle back and really pull it out in a game like that is huge," Davis said. "I gave a little fist bump to let you know how much this win meant to us."
Zimmerman hit solo homers in the first and fourth innings before adding a two-run drive in the fifth, all off Chris Tillman. With a chance to tie the major league record of four homers in a game, Zimmerman came to the plate in the seventh against Steve Johnson and looked at a third strike.
It was the first three-homer game of Zimmerman's career. He came in with three home runs in 38 games this season.
"It's tough to have a night like that and not be able to kind of celebrate or have fun with it because we lost," Zimmerman said. "I've been kind of swinging the bat better. Tonight, everything kind of came together in one game."
Davis hit a solo drive in the fourth and capped the pivotal seventh with his major-league leading 19th homer, a two-run shot off Tyler Clippard.
"He's been doing that the whole year, so every home run he hits from now on is just another one," teammate Manny Machado said. "It's impressive pop. He's a tremendous hitter and he's swinging the bat well."
Davis is batting .359 after going 18 for 30 over his last seven games.
"It's fun," he said. "You're always working on things, whether you're locked in or struggling. Just the consistent approach every day has helped me out."
Steve Johnson (1-1) pitched 2 1/3 innings and Jim Johnson worked a perfect ninth for his 16th save.
Baltimore trailed 6-3 in the seventh before rallying against Jordan Zimmermann (8-3), who was seeking to become the majors' first nine-game winner.
After Ryan Flaherty singled and Steve Pearce homered, Nate McLouth got an infield hit and scored on a double by Machado. Clippard entered and gave up an RBI single to Nick Markakis before Davis crushed a 3-2 pitch to make it 9-6.
"It's on me. I am too good a pitcher to let that happen," Clippard said. "You have to execute the pitches you need to throw in those situations."
Zimmermann gave up seven runs and 10 hits in six-plus innings. He came in with a 1.71 ERA and a streak of 15 straight starts allowing three runs or less.
He, like Clippard, shouldered the blame for the defeat.
"Those guys give me six runs like that and I ought to do a better job. We should win this ball game," Zimmermann said. "It's solely on me this time."
Tillman gave up six runs and eight hits -- including a career-high four homers -- in 4 2/3 innings. He left with Baltimore down 6-2.
In his defense, it was a tough night to be a pitcher.
"Strange things happen in this ballpark," said Nationals manager Davey Johnson, who held the same job with the Orioles in 1996-97. "It's a great ballpark to hit in, but it's a tough park to pitch in."
The major league record for home runs in a game is four. The last to do it was Josh Hamilton, for Texas on May 8, 2012, at Camden Yards, of all places.
After Zimmerman homered in the first inning, Baltimore tied it in the second. Davis singled and scored on an RBI double by Flaherty, who was recalled from Triple-A Norfolk earlier in the day.
In the Washington fourth, Roger Bernadina sent a shot onto Eutaw Street beyond the right-field wall and Zimmerman followed with a drive to center that traveled an estimated 430 feet. After going homerless in the first 36 games of the season, Bernadina has connected in two straight.
Davis got the Orioles to 3-2 in the bottom half, but Zimmerman's third homer of the game highlighted a three-run fifth for Washington. Markakis homered with no one aboard in the sixth for Baltimore.
Coming into the game, Zimmermann had allowed only three homers in 10 starts, no more than one per game.
Baltimore optioned INF Yamaico Navarro to Norfolk to make room for Flaherty. ... Washington 2B Danny Espinosa (wrist) returned after a five-game absence. ... Nationals RF Jayson Werth (groin) was scheduled to begin his rehab assignment with Single-A Potomac on Wednesday night. ... Baltimore RHP Freddy Garcia faces Dan Haren in the series finale Thursday night. ... Davis has scored a run in eight straight games, three short of the team record set by Frank Robinson in 1966.