Robert Gsellman sent the Mets in the direction of another loss before they could lug one bat to the plate against Atlanta ace Julio Teheran.

The Braves got to the 23-year-old rookie for five runs in the first and went on to claim an 8-2 victory last night at Citi Field, handing the Mets their fifth straight setback and ninth in 10 games.

Gsellman allowed six of the runs, five of them earned, a career-high 10 hits and three walks in four-plus innings. He threw 100 pitches, only 59 for strikes.

His count for runs allowed in the first is up to 10 after four starts. It was the third time he didn’t make it past five. And his velocity was down in the fifth when he gave up three straight singles to open the inning and departed.

“Tonight, I didn’t think his stuff was as crisp as we’ve seen it,” Terry Collins said. “His velocity was down a few ticks from what it normally is. He’s usually got better command of his off-speed pitches . . .

“I think after the first inning, he gets a better feel for his sinker. It hasn’t been as sharp as we’ve seen it. Hopefully, there’s nothing wrong physically. But we’ve seen better stuff.”

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In the first, he threw a 93.4 mph two-seam fastball to Nick Markakis. In the fifth, he threw him 90.5 and 91.5 two-seamers.

“It was just down,” Gsellman said. “Just being in the cold, a little tight. It’s all right. It’ll be back the next start.”

Tight?

“Just mechanically tight all around, not staying loose,” Gsellman said. “I’ll fix it.”

Last September, Gsellman helped save the Mets’ playoff hopes, going 3-1 with a 2.06 ERA in six starts. Now he will finish April at 0-2 with a 6.23 ERA, including one scoreless inning of relief.

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His previous start, against the Phillies, was his lone quality start. This one was low on quality.

He walked Ender Inciarte to open the game. Brandon Phillips followed by singling Inciarte to third. Phillips tried to swipe second but came off the bag and was tagged out. Gsellman, though, walked Freeman on a full count.

Then Matt Kemp poked an RBI single into right. Markakis delivered an RBI single to left. And Gsellman made a throwing error to first on an Adonis Garcia dribbler — bases loaded.

Tyler Flowers cleared them, lining a double down the rightfield line — 5-0. It took Gsellman 36 pitches to get three outs.

“Just location of the fastball,” catcher Travis d’Arnaud said.

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Gsellman again blamed mechanics for his first-inning failures.

“It always is,” he said. “I was just flying open and the ball was running side to side again instead of down.”