NEW YORK -- Amed Rosario swung down on the ball, beating it into the ground as the stopwatch clicked on. The Mets’ fastest player chucked his bat, dropped his head and sprinted.

At shortstop, Trea Turner edged forward, thought better of that plan and took a healthy step back, fielding the ball on a high hop. Rosario churned his legs, pumped his arms. Jeff McNeil ventured as far off second base as he dared -- just far enough to force Turner to loft his throw high over McNeil’s head. Rosario sprinted, trying to increase his speed. First baseman Howie Kendrick reached out with his glove to catch the ball. Rosario lunged with his left leg, slamming his foot down on the bag.

Total time elapsed: 4.03 seconds.

• Box score

“Oh, man,” teammate J.D. Davis said. “We were screaming at him, ‘Run! Run! Run!’”

When umpire Ryan Blackney spread his arms wide, signaling safe, the Mets piled out of their dugout and screamed some more. Rosario’s four-second dash gave him an RBI, walk-off infield hit, leading the Mets to a 6-5 win over the Nationals that could have turned into an extra-inning slog or, worse, a loss in a game during which the Mets trailed in the eighth. Given that turn of events, they poured onto the Citi Field grass, surrounding Rosario at second base. Pete Alonso trailed behind his teammates, grabbing a cooler from the dugout and hauling it onto the field, where he splashed its contents across Rosario’s back.

“You’ve got to throw the Gatorade on someone to celebrate a walk-off, right?” Alonso said. “There’s no other way.”

For a Mets team that had lost five straight heading into this homestand, falling into such disarray that general manager Brodie Van Wagenen scheduled a press conference to discuss manager Mickey Callaway’s job status, the victory was critical. And it almost didn’t happen.

The Mets and Nationals scored one run apiece over the game’s first six innings, before the curtains drew on an entirely different act in the seventh. After Brian Dozier hit a two-run homer off Zack Wheeler to give Washington the lead, Davis yanked it back with a pinch-hit, three-run shot in the bottom of the inning. The Nationals scored twice more in the eighth to take a one-run advantage, then Alonso lofted a game-tying blast over the foul pole in left.

Back and forth the advantage went, until Nationals rookie Tanner Rainey -- pitching in a tie game on the road instead of lights-out closer Sean Doolittle -- walked two batters with one out in the ninth. McNeil followed with a fielder’s choice, bringing Rosario to the plate.

Not so long ago a rookie himself, Rosario has become a mentee of double-play partner Robinson Canó, who came under fire last weekend for failing to run out multiple ground balls in Miami. Callaway made sure to address the issue with Cano, who apologized to both his manager and his teammates. On Monday, Callaway said publicly that the lack of hustle was unacceptable -- not just for Cano, but for everyone in blue and orange.

“We did have to have a conversation, an internal conversation with Robbie,” Callaway said. “But there was no worry that it was going to affect Rosie or anybody.”

To the contrary, Rosario sprinted out of the box at a faster clip than most Major League players are capable. His top speed of 30.5 feet per second cleared the threshold of what Statcast data considers elite, prompting Callaway to say afterward that he believed Rosario tapped an extra burst of speed as he neared first base. Rosario concurred.

“I don’t know if it was because of the situation of the game,” Rosario said through an interpreter, “but I felt like I got into another gear.”

In his head, Rosario had one thought: beat the throw at all costs. At second base, McNeil danced once, twice, three times off the bag, locking eyes with Turner and hoping to distract him. At shortstop, Turner made the split-second decision to play the ball on a full hop instead of a short one, giving Rosario the extra time he needed to win the Mets a second straight game.

“I know it’s only two wins,” Davis said, “but it’s a big two wins for us right now. We’re staying positive and we’re having fun.”