Gio Gonzalez allowed three hits and two walks as he fired a career-high 84 strikes against the Mets. (Jim McIsaac/GETTY IMAGES)

The maddening first half of the Washington Nationals’ regular season ended Sunday afternoon with a pulverizing reminder of how good they can be, their sine-wave tendencies on full display. The day after a journeyman deadened their bats, the Nationals thrashed the New York Mets’ prized pitching prospect. One of their three aces carved seven scoreless innings as their defense atoned for three Saturday errors with flawless fielding. They erupted, not bothering to wait a day for the return of Bryce Harper.

In a 13-2 demolition at Citi Field, the Nationals received their largest offensive output of the season as Gio Gonzalez continued his recent brilliance. The Nationals thumped high-profile rookie Zack Wheeler for 42 / 3 innings and pounded the Mets’ bullpen so severely they sent backup catcher Anthony Recker to the mound in the ninth inning. Ian Desmond launched one of Recker’s batting-practice fastballs to the upper deck in left field, completing the Nationals’ four-homer, 10-extra-base-hit onslaught.

The walloping nudged the Nationals’ record to 41-40 as they hit the halfway mark, back above .500 — again — and 6½ games behind the first-place Atlanta Braves. The preseason favorites have taken a bumpy path, but they have won seven of 11 games, and they do not mind the need for an uphill climb.

“I like where we’re at,” right fielder Jayson Werth said. “We’re in good stalking position. Of course it’s easier just to go wire to wire and not be put to the test. You’re almost better heading into the playoffs having to play a bunch of meaningful games in September, games you got to win, coming from behind. I think that sets you up better for the postseason.”

The Nationals have proved a single victory will not catapult them out of their season-long muddle. But one player might. The Nationals have played without Harper since May 26, a span of 31 games during which they have gone 15-16. On Monday at Nationals Park, he is expected to rejoin them.

The Nationals have been average or worse in most every circumstance except one. With Harper in the starting lineup, they are 25-18, a 94-win pace over a full season.

“Just his presence, I think it gets everybody going,” center fielder Denard Span said.

On Sunday, they drilled the Mets while Harper watched from home. Every starting position player chipped in at least one of the Nationals’ 13 hits and one RBI. Desmond, Werth, Adam LaRoche and Kurt Suzuki smashed home runs, and Span went 3 for 5 with two doubles.

“It’s a boost, especially since we haven’t seen it a whole lot this year,” LaRoche said. “We know we’ve got the lineup that can go out any given day and put up 15 hits and 10 runs, and it just hasn’t happened. This isn’t going to happen every day, obviously, but it should a lot more often than it has.”

Manager Davey Johnson has pleaded with Nationals batters all season to be more aggressive hunting fastballs. Against Wheeler, Span said, the Nationals planned to spit on off-speed pitches until he proved he could throw them for strikes. When he could not, the Nationals feasted on fastballs. They belted five of their six hits off Wheeler against his fastball, which sat in the mid-90s, including two home runs and two doubles.

“It’s a stride in the right direction,” said Desmond, whose 28 RBI in June set a team record for a month. “I think we’re capable of more than this.”

It was all overkill for Gonzalez, who earned his fifth win. He allowed three hits and two walks as he fired a career-high 84 strikes. The Mets swung at and missed 17 of his 119 pitches, his most whiffs in any start this year.

The Post Sports Live crew discuss whether Dan Haren has made his last start for the Washington Nationals after being placed on the disabled list and a disappointing start to the season. (Post Sports Live)

“That’s still a shock to me to get that many strikes,” Gonzalez said.

The key to Gonzalez’s gem came in the first. After he walked David Wright to put two runners on base with one out, Suzuki marched to the pitcher’s mound. He kept his mask on and moved his face within inches of Gonzalez’s.

Suzuki has caught Gonzalez since his rookie season, when both played for Oakland. “ ’Zuk is like Gio’s babysitter,” third baseman Ryan Zimmerman said. Gonzalez has a carefree demeanor, but at times emotions can sap his focus on the mound. Suzuki knows when Gonzalez needs a pep talk and when he needs to be chewed out. This was a chewing out.

“That’s just knowing each other,” Suzuki said. “He doesn’t take it personally. He knows.”

After the chat, Suzuki patted Gonzalez on the backside and jogged back behind the plate. Gonzalez responded with perhaps his most dominant pitching this year. He struck out Marlon Byrd swinging at a high fastball and escaped with a weak fly from Josh Satin. Gonzalez retired eight straight after Suzuki’s visit until Wright doubled to lead off the fourth. Gonzalez stranded Wright and went on to retire another nine hitters in a row.

“He wasn’t nibbling around. He was going right at you,” Suzuki said. “That’s when he does best.”

Since April ended, Gonzalez has a 2.19 ERA over 11 starts with 68 strikeouts and 26 walks in 73 innings. Opposing hitters have batted .171 against him over that span. Push aside a few rough outings early in the season, and Gonzalez has been the same starter who finished third in the 2012 National League Cy Young vote.

Gonzalez’s continued excellence will be pivotal for the Nationals to catch the Braves. They have endured an uneven season so far, but they also know, as Zimmerman said, “it could be worse.” Despite injuries and an underperforming offense, Sunday provided plenty of evidence that the Nationals can make the second half far more enjoyable than the first.

“We’ve gone through a lot of things, and we’re still in position to make a run,” Zimmerman said. “Which is all you can ask for.”