WASHINGTON — Upon completing his bullpen session Friday, Max Scherzer pumped his fist and high-fived everyone in the vicinity.

The Mets’ co-ace came through well in his return to a mound following neck spasms that have represented one more hurdle to his season, but one that he hopes he has cleared.

“Finally feel like I’ve turned the corner on this,” Scherzer said before the Mets opened a series at Nationals Park.

After throwing about 30 pitches, Scherzer is expected to return to the rotation Sunday for the first time since May 3.

Neck spasms, which he has been managing for years, forced him to miss a turn and had not allowed him to throw much before Friday.

Previously, he had been set back by discomfort below his right scapula, an issue he told The Post’s Mike Puma last weekend that he is “trying like hell” to fight through to avoid the injured list.

A few days later, Scherzer said his back feels “good.”

“Having this neck flare up a little bit gave more time to the back,” Scherzer said. “Hopefully I can finally get through everything and actually get out there, get pitching and get in the swing of things.”

The beginning of the season has been tumultuous and eventful for Scherzer, who has started five games, posted a 5.56 ERA and was suspended for 10 games following his ejection for having a suspected foreign substance on his hand during a game.

Scherzer and manager Buck Showalter expressed optimism that Sunday — presuming the righty feels healthy Saturday and gets the start — could finally begin a rhythm and routine for the right-hander.

“All this at one time,” Scherzer said. “Hopefully this will be the last of it.”

If Scherzer starts Sunday, he will not be followed by Justin Verlander on Monday.

Rather than bringing Verlander back on normal rest, the Mets are opting to give the 40-year-old an extra day, which Showalter called “prudent.”

Verlander threw 104 pitches in a win Wednesday in Cincinnati, his second start of the season.

David Peterson is expected to get the ball instead Monday in the series finale against the Nationals.

Carlos Carrasco (right elbow inflammation) is set to make his second rehab start Sunday with Double-A Binghamton. The righty threw 33 pitches in three innings for Binghamton on Tuesday.

“We hope [Sunday] goes well and then he can rejoin us,” Showalter said.

Brooks Raley (left elbow inflammation) will be activated this weekend, Showalter said, either Saturday or Sunday.

“The returns right now have been real positive on him,” Showalter said of the lefty, who struck out the side in his one rehab outing with Binghamton. “Gives us another guy that can pitch in some of the higher-[leverage situations].”