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Fountains of Wayne frontman Adam Schlesinger’s loved ones had been “optimistic” that he was on the mend before he succumbed Wednesday to coronavirus complications, according to longtime collaborators.

“This came out of the blue,” said J.J. Sedelmaier, an animator who worked with the New Jersey native for over 20 years.

The 52-year-old Grammy-winning songwriter had no underlying medical issues and appeared healthy just three weeks before his death at an upstate New York hospital, friends and colleagues told The Post.

On Thursday, Schlesinger’s longtime love, Alexis Morley, spoke out about her grief, saying she was “so heartbroken and afraid.”

“I can’t imagine life without you by my side. I love you so so much, my Adam,” she wrote on Instagram.

Sedelmaier said that, in the days before the musician’s death, Morley had been sending email updates about his health from the hospital, where he was on a ventilator.

“She was being very clear and trying to stay positive, but also saying that it wasn’t indicative of anything because nobody knows what’s going on with this [illness],” Sedelmaier told The Post.

Sedelmaier said the final email update on Monday was “cautiously optimistic” about how Schlesinger was responding to treatment.

“Everyone was taking a cautious breath and going ‘OK that seems to be going in the positive direction,'” Sedelmaier said.

But the next day was “radio silence,” and he soon learned on the news that the artist had died from complications of the virus.

“Everything seemed to have happened so quickly,” Sedelmaier said.

It’s not known how Schlesinger, who is survived by his two young daughters, contracted the dangerous disease.

Music producer and composer Steven M. Gold who has known Schlesinger for more than 26 years, said that Schlesinger left New York City for his home in upstate New York to ride out the pandemic “early on” when cases started being detected in the US.

Schlesinger started feeling sick with cold symptoms and a fever around mid-March and decided to go to a drive-thru testing center for the virus, he said.

But before he got back his results, the artist began experiencing shortness of breath and admitted himself March 23 to a hospital, where they confirmed he had the virus, according to the producer.

“He had four full days of improvement from the coronavirus-proven drugs that were administered, which he got pretty quickly,” Gold said. “Every day he was getting better.”

Gold said it was “shocking” to get the call late Wednesday that his longtime friend and colleague died from complications of the virus.

“He didn’t have any underlying issues,” Gold told The Post. “He was very, very energetic.”

Gold — who worked with Schlesinger on the TV show “Crazy Ex-Girlfriend,” where he was a songwriter — said it was that energy that fueled Schlesinger’s career-long drive to create music not only as a member of Fountains of Wayne, but also on the theater stage, for comedy specials and on television.

“The greatest thing about Adam is that he has the most incredible energy, this great physical energy that I believe came from amazing creativity where, at seven in the morning, he would jump up and get to the piano and start writing,” Gold told The Post.

He said Schlesinger, as was typical of him, had a number of projects in the works at the time of his death, including his collaboration with Sarah Silverman on a stage adaptation of her memoir, “The Bedwetter.”

“The thing I’m most upset about that is he had so much more to give and so much more music that he could’ve put out there and he had years of amazing songs,” Gold said. “And it’s now called back catalog, which is very sad thing for me as opposed to the potential for [an upcoming project].”

Silverman, who worked with him for nearly seven years, was among the many celebrities, including Fran Drescher and Tom Hanks, who paid tribute to him Thursday on social media.

“I’m in shock,” she wrote on Instagram, adding that she had just seen him three weeks prior. “I’m just kind of numb, and I can’t seem to convince myself it’s true. My body won’t believe it.”

She said that Schlesinger was like her “brother,” describing him as “hilarious and generous and kind.”

“Adam was an astonishing songwriter and composer and one of the funniest people you could know,” she said. “His songs are funny and poignant, and often — despite himself — stunningly heartfelt.”

Schlesinger won an Emmy Award for the song “Anti-Depressants Are So Not a Big Deal,” last year alongside frequent collaborator Rachel Bloom which was used in her show “Crazy Ex-Girlfriend”. He won a Grammy in 2003 for best comedy album for his work on A “Colbert Christmas: The Greatest Gift Of All!”