"That's why I tell you guys I can't figure it out. You don't know what's going to happen," manager Ron Roenicke said.
It could have just as easily gone the other way for Milwaukee with Marco Estrada (7-4) on the mound. He had given up a major league-worst 24 homers this year.
Instead, Estrada easily outshined Strasburg and gave up just two hits in pitching effectively into the seventh. He allowed four walks but didn't give up a homer for only the second time this season.
Estrada's only other outing without a homer was during a 6-1 loss to St. Louis on April 15. On Wednesday, fans showered him with a standing ovation after he got lifted in the seventh.
"The way the game was going, I'm sure everyone was really excited about it -- but to get some cheers in that game, it made me feel really good," Estrada said.
Strasburg (6-6) was tagged for eight hits and seven runs in 4 2/3 innings. It matched a career-worst outing for Strasburg, who also allowed seven earned runs in a two-inning start on July 12, 2013, in an 8-3 loss to Miami.
Braun, Lucroy and Ramirez got the day off after Tuesday night's marathon contest and yet the Brewers still pounced on Strasburg. Gennett turned on a 2-1 fastball for his first career grand slam and fifth homer of the season that landed on the concourse above right-center field.
"I always try to be on time for the fastball. He got me my first at-bat pretty much on the same pitch, and I was able to make the adjustment," Gennett said.
Davis opened the bottom of the fourth with a no doubt-about-it shot above the "Goodwill" sign in left-center for his team-high 14th homer to make it 5-2. Davis swung at a first-pitch fastball.
Strasburg said he was having issues with his mechanics that was causing hitters "to see the ball a lot better. ... Still trying to figure that out. It just doesn't feel the same, it doesn't look the same."
Manager Matt Williams said Strasburg's changeup was off. Without that pitch to throw hitters off, the aggressive Brewers could sit on fastballs.
"If you get behind, you can get in trouble," Williams said.
Davis finished with three RBIs in a game in which nearly everything went right for Milwaukee. Carlos Gomez, who finished 3 for 4, could only smile at first after his slow roller down the third-base line stayed fair in the sixth and allowed Gennett to score from third for a seven-run lead.
The Brewers' bats broke out after scoring just two runs in 25 innings in the first two games of a series between NL division leaders.
The benches did empty briefly after Gomez slid through second base on second baseman's Kevin Frandsen's relay on a double play to end the eighth inning. Gomez had reached after getting hit by a pitch.
Shortstop Ian Desmond took issue with the slide in a seven-run game and exchanged words with Gomez. No punches were thrown and the teams returned to their dugouts.
"You get hit with a ball, you get a little adrenaline going, and you're going to go in hard," Roenicke said. "It was a clean slide, it was right at the bag, so I didn't have a problem with it."
Nationals OF Bryce Harper (left thumb) has joined Class AA Harrisburg for the next phase of his rehab stint for a left thumb injury. ... Williams said C Wilson Ramos (right hamstring) could rejoin the team in time for the start of Thursday's four game series in Chicago against the Cubs. ... Doug Fister (6-5) will start the opener of the Cubs series. Milwaukee opens a four-game set on Thursday at Miller Park with Wily Peralta (8-5) taking the hill.
Follow Genaro Armas at http://twitter.com/GArmasAP