PITTSBURGH — Juan Nicasio was pretty much a one-pitch pitcher, and the fastball was all he needed against the slumping New York Mets’ offense.

Nicasio retired his first 12 batters and got his second win in six starts, and the Pittsburgh Pirates beat the Mets 3-1 on Tuesday night to complete a doubleheader sweep.

The Pirates also won the first game 3-1 behind seven scoreless innings from Jon Niese against his former team. Josh Harrison had three hits and Jordy Mercer hit his first home run at PNC Park since Aug. 31, 2014 in the first victory.

Nicasio (5-4) allowed a run on three hits and two walks over five innings. He had gone 1-2 with a 6.23 ERA in his previous five starts, and manager Clint Hurdle acknowledged last week that he was considering moving the right-hander to the bullpen.

“The rhythm was good the entire evening,” Hurdle said. “For a guy that threw 89 pitches, 75 or more were fastballs. He didn’t throw very many sliders or very many changeups. He just threw the fastball. He had some swing-and-miss stuff. It was a strong outing.”

Jacob deGrom (3-2) is winless in his last seven starts after he allowed three runs in six innings with nine strikeouts and no walks in the second game. He has not won since April 30 against San Francisco despite allowing three earned runs or fewer six times.

The Pirates had lost five of six prior to the twinbill sweep — the doubleheader was necessitated because of a rainout on Monday night.

The Mets have lost seven of their last 10 games and scored just 24 runs in that span. They are missing third baseman David Wright, first baseman Lucas Duda and catcher Travis d’Arnaud because of injuries.

“One thing we have not really been doing is applying some of the stuff we’re working on,” Mets manager Terry Collins said. “We’re striking out too much. We need to make better contact.”

John Jaso, Jordy Mercer, Sean Rodriguez and Stewart had two hits each in the nightcap for the Pirates.

Mark Melancon pitched a perfect ninth, duplicating his first-game performance, for his 19th save in 20 opportunities.

In the opener, Niese (6-2) gave up four hits, walked two and struck out two. The lefty is 3-0 with a 1.74 ERA in his last five starts.

He was drafted by the Mets in 2005 and spent his entire career in the organization, including eight seasons in the major leagues, before being traded to the Pirates on Dec. 9 for Neil Walker.

“It was a little weird because it’s the only organization I knew,” Niese said. “All those guys are great over there. I got along with all of them. I definitely miss them, but I’m enjoying my time here, too. I had a good game plan going on. I tried to live at the bottom of the strike zone.”

The Pirates snapped left-hander Steven Matz’s seven-game winning streak.