Ryan Zimmerman, center, on this pivotal play in which the Royals took the lead in the eighth inning: “I thought [Ian Desmond] would throw it to second. He always talks about coming my way, just in case. I just couldn’t get back in time.” (Jamie Squire/GETTY IMAGES)

On Sunday afternoon, in the middle of their surge back to the outskirts of the postseason picture, the Washington Nationals broke down. If they want September to mean much of anything, every chance must be cherished and cashed in. They cannot do something big — like crash the wild-card race — if they cannot do something small, like cover a base.

The Nationals spent most of their 6-4 loss to the Kansas City Royals at Kauffman Stadium scrapping their way back into the game, and then they spent the ruinous eighth inning frittering away a chance to win it. The Royals scored the go-ahead run without a ball leaving the infield, taking advantage of two grounders that could not be fielded cleanly and two bases left uncovered. A game that included Dan Haren’s resilient start, Denard Span’s four hits and Bryce Harper’s game-tying home run unraveled as a defensive meltdown snapped a five-game winning streak and spoiled the end of a galvanizing, 6-4 road trip.

“We gave it away,” Manager Davey Johnson said.

So the grounders that Adam LaRoche and Ian Desmond could not turn into outs may not have been backbreaking moments. But the Nationals will have only so many chances to gain ground on the Cincinnati Reds, who lost and still hold an 81 / 2 -game lead over the Nationals for the second National League wild card.

With Tyler Clippard off after pitching the ninth inning of a five-run win Saturday night, Craig Stammen relieved Haren in the eighth and got two quick outs. After a walk, Billy Butler squibbed a groundball off the end of his bat, straight at LaRoche.

“That’s one of my least favorite plays, is a cue shot from a righty, for the simple fact it’s not something you can work on,” LaRoche said. “There’s no way to really mimic it.”

The ball deflected off LaRoche and trickled to his right. Initially, he thought the ball had kicked left, but he scampered over to gather it. LaRoche turned to flip to Stammen but saw no one; Stammen had remained on the mound rather than rushing to cover.

“I just froze,” Stammen said. “When it hit him and bounced away, it was like, go after the ball or go after the base? Obviously, I need to go to the base.”

LaRoche still had time to outrun the glacial Butler. Flat-footed, he looked at second instead but had no chance to retire Eric Hosmer. By the time he finally rushed back to first base, it was too late.

“If I just cover first, we’re probably out of the inning,” Stammen said. “It’s kind of tough to take.”

Stammen pitched carefully to Mike Moustakas and walked him to load the bases, and Salvador Perez lumbered to the plate. Perez hit a one-hopper to shortstop Desmond’s backhand. Desmond didn’t field it cleanly, but he recovered with enough time to choose his throw.

Desmond’s arm strength may have given him a shot at retiring Perez at first.

“For me to come up and throw it across the infield, that’s a pretty risky throw,” Desmond said.

As he scooped the ball, Desmond noticed Butler glance at him. Rather than the conventional play — turning and firing to second — Desmond quickly decided his best chance was trying to retire Butler at third.

There was a problem. Third baseman Ryan Zimmerman had started the inning playing deep and off the line. When Perez hit the soft liner, he had stepped toward the hole. As Desmond fielded it, Zimmerman tried to scamper back to third.

“I thought he would throw it to second,” Zimmerman said. “He always talks about coming my way, just in case. I just couldn’t get back in time.”

Zimmerman lunged at Butler, but his tag arrived a second too late. The Royals had pulled ahead.

“It wasn’t the first place I thought he was going to throw the ball,” Zimmerman said.

“I don’t know exactly what his thought process was,” Desmond said. “But it was just a mistake.”

David Lough added an RBI single for posterity.

Singles from Span and Jayson Werth brought Desmond to the plate as the go-ahead run in the ninth. Desmond earlier had slammed a 431-foot solo homer off the Royals Hall of Fame for the Nationals’ first run off starter Ervin Santana. Now he struck out swinging at closer Greg Holland’s wicked, 87-mph slider.

Seven of the first nine hitters Haren faced reached base, including a leadoff homer by Alex Gordon, and the Royals took a 4-0 lead in the first. The oppressive heat stunned Haren. After the first, he cooled off in the Nationals’ clubhouse with ammonia towels and gathered himself.

“The game can go one [way] or another right there,” Haren said. “I was determined to keep the team in the game and throw as many zeroes as I could. . . . I just couldn’t catch my breath, really. I was able to reestablish my tempo after the first inning.”

Haren retired 16 of 18 batters to finish his start, pitching seven innings. Haren’s recovery kept the Nationals close, and then a three-pitch torrent pushed them into a tie.

Santana flummoxed the Nationals for most of the afternoon, dominating right-handed hitters with a vicious slider. One out from handing off to the high-powered back end of the Royals’ bullpen, Santana faced Span.

Late Saturday night, Span received a text message from his mother imploring him to swing at the first pitch more often. “I was [ticked] off,” Span said. “I didn’t even reply back to her.”

Mother knew best. Santana tried a first-pitch change-up, and Span smoked it to right-center field. His line drives usually find the gaps, but this one sailed until it cleared the fence, his third homer of the season.

Zimmerman followed with a single on the first pitch he saw. Harper had been taking swings in the tunnel and had no idea Span and Zimmerman had smoked first pitches. Still, he cranked a first-pitch, 93-mph fastball over the left-center field fence.

“I finally got something I could drive, and I did something with it,” Harper said.

In three pitches, the Nationals had mustered a three-run rally out of nothing, knocked out Santana and tied the score. They had given themselves a chance. One inning later, they gave it away.