New York Mets pitcher Noah Syndergaard looks on from the...

New York Mets pitcher Noah Syndergaard looks on from the dugout after giving up three runs against the Arizona Diamondbacks during the fourth inning at Citi Field on Thursday, Aug. 11, 2016. Credit: Kathleen Malone-Van Dyke

Noah Syndergaard said he felt like his old self early in Thursday’s matinee at Citi Field. Instead, it was more of the same for the righthander, who has struggled since late June.

Syndergaard allowed three runs, eight hits and a walk and struck out six in five innings in the Mets’ 9-0 loss to the last-place Diamondbacks. The three-game sweep dropped the defending National League champions to .500 for the first time since April 20.

In five starts since the All-Star break, Syndergaard is 0-3 with a 3.45 ERA, allowing 11 earned runs in 28 2⁄3 innings. He is 9-7 with a 2.75 ERA overall.

“I’m aware what I’m mentally doing wrong, I just continue to do it,” Syndergaard said. “It’s kind of frustrating. I’m going out there and trying to purposely miss bats, as opposed to just going out there and making my pitches.”

Although he felt good early in the game, evidence suggested that the same things that have hurt him — high pitch counts and stolen bases — again would be a problem.

Syndergaard needed 21 pitches to maneuver out of a jam in the first. With runners on second and third and one out, he got Jake Lamb to pop to third and struck out Chris Owings.

He found himself in trouble again in the third. With runners at first and third and two outs, he again got Lamb to pop out.

More trouble came in the fourth, when Arizona finally broke through. Owings led off with a double, moved to third on Brandon Drury’s flyout and scored on Socrates Brito’s ground-rule double. One batter later, Tuffy Gosewisch’s drive down the rightfield line eluded Jay Bruce for an RBI triple. Pitcher Braden Shipley singled to make it 3-0 and proceeded to steal second without a throw.

A pitcher stealing a base, particularly without a throw, is an unusual occurrence that spoke volumes about Syndergaard’s ongoing trouble holding runners close. He allowed four stolen bases in five innings.

But that problem is not exclusive to Syndergaard. Arizona stole 13 bases in the series, a Mets record for a three-game series. It also was the most steals in a series by any team since the Rangers had 13 in three games against the Angels in August 2013.

“They’re a great team with good baserunners all around the lineup,” catcher Rene Rivera said. “This is going to happen. They picked stuff up off our pitchers and took advantage of it. It’s nothing new in baseball.”

Arizona (85) ranked fourth in the majors in stolen bases entering play last night. San Diego (99), which visits Citi Field this weekend, ranked third.

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