SAN DIEGO — Tyler Clippard had already squirmed out of another teeth-clenching jam when he emerged for the eighth inning Thursday night. He had already experienced the kind of strain that had become his new norm this season. He would labor for more outs as a victory slipped away, a capable bullpen of fresh relievers just waiting for the call from Manager Davey Johnson.

But Johnson continued to maintain a singular thought: There was no one he would rather have on the mound than Clippard.

The Washington Nationals asked Clippard for one more escape act, and for once he could not deliver. After Johnson stuck with Clippard for a second inning, the San Diego Padres stunned Clippard with two runs in the eighth and beat the Nationals, 2-1, to prevent Washington from completing a sweep before heading into Dodger Stadium for a showdown between the National League’s best two records.

Edwin Jackson dueled Padres starter Edinson Volquez for much of the night, the only damage off either pitcher coming when Jayson Werth drilled a solo home run in the seventh inning to give the Nationals (14-5) a 1-0 lead. Jackson’s 6 ² / ₃ scoreless innings made the Nationals the first team since 1900 to receive scoreless starts in eight of their first 19 games.

The Nationals fell to 6-3 in one-run games. They had come to believe they would find a way to snuff every threat and generate a clutch hit when needed, no matter what happened during the first eight innings. Thursday night, as they managed only four hits, the Nationals could not do it again.

“We expect to win those close ballgames,” Clippard said. “It was a pitchers’ duel, and we got the situation we wanted to be in with the lead late in the game. That’s the kind of team we are. We understand that. We’re all confident in close games. We’ve won them all year.”

This night would unfold differently. Clippard relieved Jackson in the seventh inning with two men on and two outs. He promptly walked Chase Headley, a sign of the command difficulty that led to him throwing 26 pitches Wednesday night.

Clippard has been gutsy this year, but not his dominant self. Over the past two years, batters missed his pitches 33 percent of the time they swung. This year, the number has dropped to 21 percent.

“They know him a little better,” pitching coach Steve McCatty said. “They’re putting better wood on the ball, fouling pitches off. They’re doing reports on him. I don’t think he’s not as good as last year. They’re just making contact a little more. They study these guys.”

But he is still the Nationals’ all-star set-up man, the pitcher who punched up a 1.83 ERA and struck out 10.6 batters per nine innings last year, the pitcher Johnson turns to in the toughest jams. After Nick Hundley smashed two change-ups foul, Clippard blew a 2-2, 94-mph fastball past him with the bases loaded. He pumped his fist on the mound, an exclamation point he had used in recent clutch escapes.

“You definitely want him with the ball at the end,” Werth said.

Clippard had yet to pitch across multiple innings this season, but he told Johnson and McCatty he felt great. Even with Ryan Mattheus and Sean Burnett fresh, Johnson stuck with Clippard for the eighth. He wanted the ball in Clippard’s hands.