A special day from Jacob deGrom turned sour with one swing of the bat.

The Mets returned the favor — and then some — two innings later to finish off a four-game sweep.

After deGrom’s 13-strikeout gem was spoiled by a game-tying home run in the sixth inning, Brandon Nimmo delivered a go-ahead single in the eighth inning as part of a four-run rally that lifted the Mets to a 7-3 win over the Pirates on Sunday afternoon at Citi Field.

“We had a tough series against the Cubs [a three-game sweep], but this group does a good job of coming in the same way every day and the goal is to win baseball games,” deGrom said. “It’s big for us to sweep this series. Now, we have to do a good job of turning the page and going to Milwaukee and trying to win a series there.”

With the win, the Mets (93-55) secured their first sweep since taking a three-game set against the Reds  Aug. 8-10 and preserved a one-game lead over the Braves atop the NL East. They also positioned themselves to clinch a playoff spot as early as Monday with a win over the Brewers.

“Our guys are continuing to earn it daily,” manager Buck Showalter said. “It’s really hard to beat a major league team four times. To do that at this time of year, really proud. Every team is a little weary this time of year, whether it’s mentally, emotionally, a little physically. You gotta keep drawing. Keep drawing, knowing there’s a pot of gold down the road if you can keep grinding.”

Tomas Nido led off the eighth inning of a 3-3 game by ripping his second single of the day to center field before being pinch-run for by Terrance Gore.

The Pirates (55-92) then brought on former Yankee Manny Banuelos to face Nimmo. Before Banuelos delivered his first pitch, the lefty threw over to first base three times to try to keep Gore at bay, but it did not work. The speedy Gore stole second on Banuelos’ first pitch and took third on the throw to second base that trickled into center field.

Nimmo then came through with a well-placed bloop single to left field, putting the Mets up 4-3.

The Mets went on to create some breathing room on Daniel Vogelbach’s two-run single off Duane Underwood Jr. before the Pirates’ fourth error of the day led to another run.

Otherwise, the day largely belonged to deGrom, whose season-high 13 punchouts were part of the Mets’ 20 — including one from Seth Lugo, a career-high five from Joely Rodriguez and one from Trevor May — that set a franchise record and tied a major-league record for a nine-inning game.

“Outs are outs, but when you think how hard it is to do it and how infrequent it has been, that gets your attention,” Showalter said.

After deGrom allowed a 111 mph double by Oneil Cruz to begin the game, he went on to retire the next 15 batters in a row, 13 by strikeout. The Pirates did not record another base runner until Zack Collins led off the sixth inning — at which point the Mets led 3-0 — with a single up the middle and Jason Delay followed with a bloop single to right field.

Cruz came up next and sent deGrom’s season-high 101st pitch of the day to the Pirates’ bullpen, crushing a three-run homer that tied the game 3-3. It ended deGrom’s day and left Citi Field stunned.

“I actually looked at it again, not a terrible pitch. I’m more mad about the [two] at-bats before,” deGrom said. “I felt good until the sixth. Didn’t make some pitches when I needed to. The guys did a good job of putting up some runs for me and I wasn’t able to go out there and keep them where they were at. But these guys do a great job and they battled the rest of the way. Ended up getting the team a win.”