The series was split. The score was even. The battle was between the bullpens.

And the Mets were no longer on equal footing.

Even on a Sunday when Edwin Diaz and Jeurys Familia improbably spun back-to-back shutout innings, the Mets’ beleaguered relief corps threw another series of daggers into the team’s playoff hopes, combining to allow six runs — after Noah Syndergaard failed to hold an early three-run — in a 10-7 marathon loss to the Phillies at Citi Field. The four-hour, 29-minute affair marked the longest nine-inning game in franchise history.

The Mets have lost eight of their past nine games at home — their second straight defeat propelled the Phillies two games in front of them — and blew back-to-back opportunities to cash in on Cubs losses, remaining four games back of the second wild-card spot in the National League with 20 games left.

“We gotta win games. You can’t lose two in a row,” manager Mickey Callaway said. “It stings, but you gotta move on. We’re still the same amount of games back. We have one less game to make that up, but we can do it.”

The climb should have been easier.

Following their first shutout loss in seven weeks, the Mets spotted Syndergaard a 3-0 lead, via back-to-back first-inning homers from Wilson Ramos and Robinson Cano.

But Syndergaard went on to deliver his second-shortest outing of his strong second half.

After surrendering a pair of runs — including a solo homer to Adam Haseley — Syndergaard was still in line for the win, when Jeff McNeil failed to catch what would have been an inning-ending pop-up near the left-field tarp in the fifth. Corey Dickerson’s second chance produced a two-run single, giving the Phillies a 4-3 lead.

Though Syndergaard had only thrown 78 pitches, the right-hander was then removed for pinch-hitter Todd Frazier, who struck out with the bases loaded to end the bottom of the fifth.

“Noah had just given up four runs and they had scored in three consecutive innings. He’s at 78 pitches and you gotta try and take the lead right there,” Callaway said.

“We’re thinking we take the lead, we get the sixth inning out of the way, and we can go to [Justin] Wilson for one [inning] and [Seth] Lugo for two [innings], and you’re gonna win a game if you can get to that seventh inning.”

The Mets bullpen happened while their manager was busy making other plans.

Paul Sewald entered and allowed a leadoff double. Luis Avilan took his place and gave up a two-run homer to Maikel Franco, giving the Phillies a 7-4 lead. Even after the Mets cut the deficit to one in the bottom of the sixth, Wilson looked like any other Mets reliever, allowing a two-out homer to Scott Kingery. Bryce Harper then added another run, drawing a bases-loaded walk against Tyler Bashlor.

“Bottom line is our bullpen had a rough night,” Callaway said.

The offense did its part, but could have done more, leaving 14 men on base. The Mets brought the tying run to the plate multiple times — three at-bats in the seventh, two at-bats in the ninth — but couldn’t come through, with Pete Alonso and Michael Conforto put down at the end of the miserable weekend.

“We can’t look at how many games are left, how many games back we are. We gotta keep winning games,” Conforto said. “That sucks, but we gotta move forward. That’s all we can do.”