The Post Sports Live crew talks about Bryce Harper’s head-on collision with the outfield wall in Dodgers stadium during Monday night’s game. (Post Sports Live)

The thing about pinch hitting is, it only takes one swing. One minute, Chad Tracy can be sitting in the Washington Nationals dugout, a bystander as victory nears. The next, he may be running to the end of the tunnel for two emergency warmup hacks. One night, he may be staying up in bed wondering when the next hit will come. The next, he could pump his fist while circling the bases, having rescued the other 24 men in his clubhouse from an agonizing loss.

Tracy and the core of Nationals pinch hitters he leads – the Goon Squad, they call themselves – had flailed through the early season. In the 10th inning Friday night, Tracy blasted Huston Street’s elevated change-up over the right-field fence at Petco Park, lifting the Nationals to a harrowing, 6-5 victory over the San Diego Padres. With one swing, Tracy struck a blow for Washington’s bench, bailed out Ryan Zimmerman after another throwing error and made the Nationals, in their 42nd game, the final National League team to record a pinch-hit RBI.

“As a collective group, we haven’t really come in and won a ballgame for us yet,” Tracy said. “Last year, we did that early on in the first series of the year. It took a little longer this year. That’s something that can get us going. We can feed off each other.”

Even after Tracy’s home run, the drama persisted. With Rafael Soriano having blown a save in the ninth, Drew Storen pitched a heart-stopping 10th for his first save of the season. He wiggled out a first-and-third, one-out jam only after a strikeout and a nearly disastrous bobble by Zimmerman.

“That one gave me a few more gray hairs,” Manager Davey Johnson said. “And I don’t need any more.”

The Nationals’ Bryce Harper’s unusal swing has been compared to the likes of baseball legend Babe Ruth. This animation shows just how similar the two players are. (TWP)

It should have been an easy night. Adam LaRoche clobbered two homers and drove in four runs, continuing to drag the Nationals’ scuffling offense to respectability. Zimmerman added one of his own, giving the Nationals a 5-2 lead after 51 / 2 innings. Starter Gio Gonzalez handed a 5-3 lead to Tyler Clippard with two outs in the sixth, and Clippard carried the ball into the ninth, to Soriano, who had been automatic. Tracy, out of habit, noticed Soriano’s spot in the order was due up third in the top of the 10th.

With one out in the ninth, protecting a 5-3 lead, Soriano induced a chopper from Chris Denorfia. The ball bounced to the left side of the infield, one of the final pieces to a victory for the Nationals. Zimmerman and shortstop Ian Desmond had consulted before the play and decided that Desmond, with Soriano throwing cutters to a right-hander, would play up the middle.

“I would never complain about a play, but that’s not an error,” Zimmerman said. “I don’t know if they have Brooks Robinson as a scorekeeper here or what. . . . I’m almost near second base and throwing across against not a slow guy. If that’s on the base, it’s bang-bang. We’re not supposed to complain about that stuff, but that one’s a little ridiculous. I guess if anyone deserves to get errors put on them right now, it’s me. But that’s over the top.

“I’m sure Denorfia wants the hit. I want the hit if I’m him. It is what it is. In the end, I feel bad because it hurt Sori. If we get that guy right there, it’s a different inning. I don’t know what else I can do on that play. If a get the ball and throw across my body, it’s not a play you practice.”

Soriano settled down, recording an out. One more, and he’d have his 13th save. Instead, Chase Headley smacked a single to right on an 0-2 pitch, scoring Denorfia with a meaningless run. Pinch-hitter Yonder Alonso kept the inning alive with another single, a grounder through the right side. Kyle Blanks shot yet another single to right. Like that, the Padres had tied it at 5.

The Nationals made two quick outs in the 10th, which continued their baffling failings against relievers – it made them 0 for 13 with seven strikeouts against the Padres bullpen Friday night. Up came Tracy. Last September, the Nationals valued him so much they signed him to a one-year, $1 million extension. So far, he had gone 6 for 37. As a discipline, pinch-hitting is diabolical. Once you go cold, you have few chances to get unstuck.

“You go sit in your hotel room, sit in your bed at night, and think about it until you get your next at-bat,” Tracy said. “When you don’t get a hit that at-bat, you think about it even longer. It just builds and builds and builds. Finally, you just let it go, and whatever happens, happens. I think we’re kind of at that point now.”

Tracy walked to the plate, telling himself not to think too much. Street started with a change-up for a ball, then painted the outside corner to even the count. Street tried one more change-up, only he left it up just enough, inside. Tracy got out in front of the pitch, and even though the ball caromed off the end of his bat he knew he had gotten enough of it.

As he rounded first, Tracy pumped his fist. In the dugout, his teammates pounded his head and smacked his back.

“You’ve got somebody that’s a leader like Trace, doesn’t play a whole lot, yet he continues to stand there and support the team all game long,” LaRoche said. “To see him come up and get his first home run in a huge way for us, it was awesome.”

Having removed Soriano, Johnson turned to Storen for the save. The past two seasons, when healthy, Storen had received all of the save chances. This year, with Soriano signed to a two-year contract, he had been setting up.

“He’s been one of the best guys for us in the back end of the bullpen for two, three years now,” Zimmerman said. “I don’t think anyone in here doubts the kind of stuff that he has.”

With one out, John Baker drilled a chopper up the middle. Storen stuck out his right handed and deflected the ball. Nationals coaches came to check on him, but he remained in the game. The ball, luckily, had hit the fat part of his palm.

“I’m not going to try to be a hero if it’s going to cost us the game,” Storen said. “I felt fine with it.”

But Alexi Amerista followed with a sharp single to right, putting runners on the corners with one out. Up came Denorfia, fast enough that even a typical double-play ball could result in the game-tying run. Storen knew he needed a strikeout.

“You’re pretty much trying to miss the bat or get something really soft or just change speeds on him. He’s a good hitter, obviously. I was trying to change-up as much as I can on him. I was happy with the pitches I threw him.”

He fell behind when he bounced a slider, which Suzuki blocked. (“Huge,” Storen said.) Storen threw a fastball that Denorfia fouled off to even the count. Denorfia would not see another fastball. Storen rarely throws change-ups to right-handed batters. With the count 2-2, he fired one that started at Denorfia’s thighs and veered away from him to ankle-height. Denorfia swung through it.

“That was a nasty pitch,” Tracy said. “Hats go off that he stayed in the moment and beared down.”

Storen then induced a spinning grounder from Everth Cabrera, right to Zimmerman. Zimmerman bobbled the ball, and the sleepless hearts back in Washington rose to their throats. He scooped the ball and, under scrutiny, fired a strike across the diamond to end the game.

“I just caught a little in-between hop, and it scooped out of my glove,” Zimmerman said. “You just have to not panic. Panic will make it worse. You just kind of grab it and make a strong throw.”

The Nationals lined up in the middle of the dugout, exchanging high fives and hugs, all because of one swing.