Danny Espinosa bobbles the ball as New York’s Daniel Murphy steals second base. (Kevin Lamarque/Reuters)

After R.A. Dickey unleashed his dark art Thursday afternoon, the Washington Nationals wondered how much control he actually exerted over it. The knuckleball pitcher for the New York Mets sacrifices control for trickery, but Dickey’s knucklers behaved in a complex, purposeful pattern. Some fluttered, some sank and some bored hard into Nationals players’ hands.

“I don’t know if he is doing it on purpose,” third baseman Ryan Zimmerman said. “But it sure seems like he is.”

Two rollicking victories preceded the Nationals’ Thursday matinee, games packed with adrenaline and action, wild moments and harebrained twists. But Dickey lulled the Nationals to sleep over 71 / 3 innings in a 3-1 loss, costing the hosts a sweep and ensuring their lead in the National League East would be down to one game after Thursday. Chien-Ming Wang kept the Nationals in the game by allowing two runs over 51 / 3 rocky innings, but Dickey’s mastery sucked the life out of Nationals Park, even with 32,096 in attendance.

As the Nationals packed their bags for Boston to play the Red Sox, their first of five consecutive series against the American League East, Manager Davey Johnson called into question whether Bryce Harper would play Friday at Fenway Park.

Johnson said Harper had a “tender” lower back, a revelation prompted by Harper getting the wind knocked out of him as he dove fruitlessly for a flyball in the ninth inning. Asked if Harper would play Friday, Johnson said: “He might not. We’ll see how he is on the trip.”

Harper, who has played in all 35 games since the Nationals summoned him from Class AAA, appeared surprised his back had become an issue. He said categorically that he would not sit out.

“I’m totally fine,” Harper said. “I’m going to play. I won’t be out of the lineup.”

Harper occasionally wraps his back in ice after games, nothing out of the ordinary for a postgame clubhouse in June. Asked when he first hurt his back, Harper replied: “I didn’t hurt it. I’m good. Don’t worry about it. Feel great.”

The Nationals instead could worry about Wang, who has not found consistency in either of his first two starts since replacing Ross Detwiler in the rotation. On Thursday, Wang yielded eight hits and three walks and also hit a batter, needing 84 pitches for 16 outs. In his first two starts, both losses, Wang has allowed 22 base runners over 91 / 3 innings.

“Right now, I just need to pitch more innings and get settled,” Wang said through an interpreter. “Once I get the feel back, I should be fine.”

Wang loaded the bases in the second and third innings and escaped each jam, the second with a double-play ball chopped right back at him. He collected enough groundballs with his sinker to keep the Mets scoreless through four innings, but a bad habit cost him when Lucas Duda pummeled a two-run homer to right in fifth.

Wang’s inconsistency rises from one issue with his delivery. As he swings his arm toward the plate, he too often throws with his hand to the side of the ball rather than on top of it. The mistake causes the ball to run across the plate rather than sink. As a result, pitches stay up and groundballs become liners.

“He sometimes gets out of the slot he needs to be in,” Johnson said. “It’s just a consistency thing.”

Thursday afternoon, Dickey’s performance would have bested Wang even on a good day. Dickey allowed four hits and a walk in 71 / 3 scoreless innings, dropping his ERA to 2.44 and improving his record to 9-1. With one knuckleball in the 80s and another that wobbled in at the mid-60s, Dickey confused and dominated the Nationals.

“He’s very good,” Harper said. “He throws it hard, throws it soft. He’s a pretty unbelievable pitcher. It was fun to face him. It wasn’t fun going 0 for 4.”

Once Dickey left the game, the Nationals created one last, best chance to make it a game. Harper came to bat in the eighth inning against Mets reliever Bobby Parnell with runners on second and third and no outs. The moment passed as soon as it materialized.

Harper grounded the first pitch to shortstop. He hustled down the line as Omar Quintanilla charged. Quintanilla’s running throw beat Harper by an inch. Harper ripped off his helmet and put his hands on his hips.

In the top of the ninth, Harper gave the park the wrong kind of drama. David Wright hit a blooper down the right field line. Harper made a long run and an ill-advised dive. The ball bounced a foot in front of him and rolled past, allowing to Wright to turn a single into a triple.

After Harper fired the ball to the infield, he collapsed to his knees. He popped back up, and head athletic trainer Lee Kuntz jogged out to visit him. Harper remained in the game.

“I just took a moment,” Harper said. “Nothing wrong with my back.”

Zimmerman gave the Nationals a measure of late life in the ninth inning, drilling his third home run of the year off of Mets closer Frank Francisco. The blast provided promise for a hitter looking to find his power and prevented a shutout, but the Nationals would still lose in familiar fashion.

The Nationals have had 10 chances to sweep a series this year, and, oddly, have gone 1-9 in those games. Of course, if they can keep doing that all season, they will play deep into October.

“It’s pretty good to have 10 chances to sweep, I guess you can look at it that way,” Zimmerman said. “It’s hard to sweep a team. That means we’ve won the series. If you can sweep, it’s nice. But winning the series in the ultimate goal. If we continue to do that, we’ll be just fine.”