WASHINGTON – Facing the last guy you want to face with bases loaded, Julio Teheran threw a pitch over the middle that sealed the winless Braves’ fate in Thursday’s series finale at Nationals Park.

Bryce Harper crushed Teheran’s fastball for a third-inning grand slam that erased a 1-0 Braves lead and sent the Nationals to a 6-2 win and four-game sweep at Nationals Park. Their nightmarish April continued for the Braves (0-9), now one loss from matching the poorest start in franchise history, covering more than 100 years of Braves baseball.

“One mistake, and it was at the perfect time,” said Teheran, and by perfect he meant perfectly awful. “You don’t want to make a mistake with bases loaded. That’s what I did, and I paid for it.”

It’s the worst start for the Braves since they lost their first 10 in a 106-loss season in 1988. They’ve dropped 14 in a row at Nationals Park since the beginning of last season.

“I would bet that out of everyone in here, no one’s been in this situation,” Braves veteran Kelly Johnson said. “This is uncharted territory, it’s not something you plan for or think about too much. I’ve got to think it’s like being in an 0-for-20 slump or something, get that one hit any way you can. I mean, if we got a win on a bases-loaded balk, I don’t care.”

Stephen Strasburg, a pitcher the Braves gave fits earlier in his career, continued a two-year winning streak against them, allowing four hits, two runs and two walks with seven strikeouts in 7 2/3 innings. He’s 5-0 with an 0.57 ERA in his past five starts against the Braves.

“God almighty, he was good,” said manager Fredi Gonzalez, whose Braves are batting .196 and have been out-homered 13-3. “His fastball had some life to it, and after the second inning he got hard to see with all the shadows. But he had all his pitches working today, and he had some life on his fastball.”

Atlanta’s minus-31 run differential is easily the worst in baseball. They were outscored 9-4 in the last two games of the series, after twice leading the series opener and losing 2-1 in game 2.

“I certainly don’t think losing those (last two) games was the same way we lost the first couple,” Johnson said. “They hit, they made some plays, they pitched really well. I mean, they were better, that’s for sure. They’re a good team, and that’s what we’re going to see this month, a lot of good teams. We’re going to have to find something that works and find ways to come together. That’s all you can do.”

Harper has eight homers and 22 RBIs in his past 21 games against the Braves. Wilson Ramos added a solo homer in the fourth and a two-out RBI single in the sixth against Teheran (0-2), who’s allowed 10 runs (nine earned) in 11 innings of consecutive losses to the Cardinals and Nationals.

After Nick Markakis gave the Braves a 1-0 lead with a two-out single in the third, Teheran had the opposite of the desired shutdown inning. He gave up four in the bottom of the third on one big swing. But his biggest mistake, in Gonzalez’s view, was walking No. 7 hitter Matt den Dekker to start the inning, after being ahead in the count 1-2.

“It can’t happen,” Gonzalez said of the walk. “We just score one run against a tough opponent and then — nothing against den Dekker, but he’s not Harper — and then we end up walking him. And that set up that situation.”

Teheran retired the next two batters before allowing consecutive two-out singles by Chris Heisey and Anthony Rendon.

With nowhere to put Harper, Teheran went after him not very artfully. Harper took a first-pitch change-up for a ball, then crushed a 91-mph fastball down the middle, banging it off the upper-deck façade in right field for his 100th homer.

Harper’s 15 homers against the Braves includes five against Teheran in 28 at-bats. Teheran has won three of 13 starts against the Nationals. He’s 0-2 with a 4.85 ERA in his past seven against them, with 10 homers allowed in 42 2/3 innings.