Mets pinch hitter Jose Reyes strikes out swinging against the...

Mets pinch hitter Jose Reyes strikes out swinging against the Nationals during the sixth inning at Citi Field on Tuesday. Credit: Kathleen Malone-Van Dyke

Sixteen games into the season, the Mets are on their first losing streak.

Zack Wheeler gave up three runs in six innings, Robert Gsellman gave up two in two and the Mets left nine runners in scoring position as they lost their second straight game to the Nationals, 5-2, Tuesday night at Citi Field. It also marked the first time the Mets lost a series this year.

Todd Frazier had his six-game hitting streak end with an 0-for-5 with four strikeouts, including the final out. He said Mets fans “shouldn’t panic” over two consecutive losses.

“It’s just two games,’’ said Frazier, who left four runners on base. “I understand, it’s against the Nationals, [but if] you sit back and say, ‘These first 16 games, you’d take 12-4?’ Any day of the week.’’

Wheeler (1-1) struggled at times but overall pitched well enough to give the Mets a chance. When he left the game, the Mets were within striking distance, down just 3-2.

“I thought he threw the ball real ly well,’’ Mickey Callaway said. “I thought the only mistake he made in the game was when he walked [Michael A.] Taylor. That probably cost him two runs. But other than that, he looked pretty good. He didn’t even have his best stuff. His strikeout, swing-and-miss stuff wasn’t there tonight, and he battled through it and got through six. And got a quality start.’’

“I did OK,’’ Wheeler said. “I made a couple mistakes when I was ahead in the count, especially there in the fourth. I could have located the ball a little bit better, but overall I think I hung in there well, and pitched out of a couple jams.’’

Gio Gonzalez allowed two runs in 5 1⁄3 innings and improved his record at Citi Field to 11-1. The Nats have won 45 of their last 61 games in Flushing.

The Mets had their chances. They had men on first and third with one out in the sixth and seventh, and two on with two outs in the eighth. But they went 0-for-5, with four strikeouts, in those situations. Jose Reyes, now hitless in his first 17 at-bats of the season, pinch hit in the sixth but struck out against reliever Sammy Solis.

“He was our switch hitter off the bench, the only righty we had, going against a lefty,’’ Callaway said. “I had confidence in Reyes in that spot.’’

Even without Daniel Murphy, Adam Eaton and Anthony Rendon in the lineup, the Nationals represented a major step up in class over the Marlins, whom Wheeler beat April 11 in his first start after being called up from Triple-A. Trea Turner made that clear when he doubled off the wall in centerfield to lead off the game. Wheeler did escape the inning without giving up a run, but the righthander needed a bases-loaded, one-out double play to do it.

Washington scored in the third, when Turner and Howie Kendrick began the inning with back-to-back singles and Bryce Harper hit a sacrifice fly to right to score Turner. The Nationals would get two more in the fourth inning, when the first four hitters went single, walk, single, single to make it 3-0.

But Wheeler managed to wriggle out of further trouble in the inning, and then used his bat to start a rally that got the Mets back in the game in the fifth. Batting eighth, he led off with a single up the middle, and Amed Rosario, batting ninth, doubled to left-center.

Glenn Sherlock held Wheeler at third on Rosario’s hit, but he sent him home on Asdrubal Cabrera’s fly ball to medium centerfield. The throw got past Pedro Severino and Wheeler slid in safely. Rosario went to third on the play and scored on Yoenis Cespedes’ slow bouncer to second, cutting the deficit to 3-2.

The Nationals love to visit Citi Field, where they’ve won 45 of their last 61 games against the Mets since the 2012 season. Their records in Flushing the past seven years:

2018 2-0

2017 8-2

2016 6-4

2015 5-5

2014 9-1

2013 7-3

2012 8-1

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