The Bronx woman killed by a falling tree in an apartment swimming pool had taken a break from work to do laps — and apparently heard the trunk crack and looked up just before being hit, the building super told The Post on Tuesday.

Donna Douglas, 59, was enjoying the gorgeous weather with a female pal at the River Terrace Apartments pool in Spuyten Duyvil just after 1:30 p.m. Monday when the horror occurred, said the worker, Alex Syku.

Syku, 57, said his 17-year-old daughter was lifeguarding at the time — on what was her birthday.

“She heard a crack. For a second, she looked up, saw the tree leaning,’’ the super said of his teen. “She yelled, ‘Oh, my God!’

“And I believe in that moment, Donna might have looked up towards the tree,’’ the man added, saying the victim’s facial injuries indicated that was the case.

“It happened at 1:34 p.m.,’’ Syku said of the tragedy. “My daughter was talking to Donna just before she was killed.

“My daughter called me at 1:35 p.m., after she helped [the other woman] out of the pool. Then my daughter called 911, and my doorman called 911, and three or four people in the building called 911.

“When I got to the pool, I could only see a little bit of Donna’s feet — she was covered completely by the tree,’’ he said.

“The pool was completely filled with blood. If you had 200 doctors, there’s nothing you could do for her.”

Syku said he dove in — fully clothed — to get to the victim.

“I pulled Donna out of the water within 3 minutes,’’ the super said.

A doctor and a retired dentist who both live in the building “ran down because they heard what happened,’’ but it was too late for anyone to help Douglas, Syku added.

Douglas’s shattered domestic partner of 20 years, Gabe Gonzalez, 70, told The Post on Tuesday that an afternoon swim was part of his beloved’s routine.

“She works from home, so she would go to the pool every day on her lunch break and swim with her friend,” Gonzalez said of Douglas, who was employed in the accounting department of the law firm Davis Polk, where she worked for 18 years.

“Then she’d go back upstairs and work, that was her routine.”

He said he was driving home from playing softball on Long Island when he got a call from the doorman, and, “[He] told me there was an accident and I should go to the pool when I get back.

“When I got there, I saw all the police. I went back down to the pool, and I was crushed,” Gonzalez said.

“They wouldn’t let me go down there and see her,” he said. “She was already out of the pool and covered.

“I’m heartbroken,” said Gonzalez, who shared cats with Douglas.

His partner, who did not have children, was “the best,” the man said. “She was beautiful. She was super smart.

“You can’t even explain it. It’s just ridiculous.”

The friend who was swimming with Douglas at the time, Beth Smith, 72, “happened to be lucky where she was between two branches of the tree, so she didn’t get any impact,” Gonzalez said.

“But she was all scratched up. … I think she’s devastated.”

Smith told The Post on Tuesday, “I’m sick, I’m hurt. 

“Nobody was at fault,” she said. “But look, here’s the thing, I have a lawyer. If you want to talk to me, you’ll have to talk to my lawyer.”   

Douglas’s death occurred two days after a Brooklyn man was struck by a falling tree branch in a local park and later died.

Syku called Douglas “one of the best residents” at The Bronx apartment complex.

“She was always smiling, happy, she loved the pool, she loved to swim,’’ he said of the dead woman, who had lived at the building for about five years and posted several photos of bucolic scenes of the ocean on her Facebook page.

A rep for Davis Polk said in a statement, “We are devastated by the death of our dear colleague and friend Donna. 

“She will be greatly missed by the entire Davis Polk community. Our deepest sympathies are with Donna’s family and friends.”

Syku said Smith has lived at the building for around 15 years and “is one of the No. 1 users of the pool.

“[She] has a bruised face but nothing major,’’ Syku said. “She came home last night at about 10 p.m. from the hospital after having CAT scans.

“She’s fine, but she’s traumatized.’’

A pink hat belonging to one of the women was still floating in the pool among tree branches Tuesday.

The city Parks Department has said the tree that fell was not on Big Apple property, making it a private owner’s responsibility.

Syku said the tree, which he estimated at 70 feet high, was on property next door to the apartment complex.

The downed tree came within a few inches of a gas pipe used to heat the pool, Syku noted, which would have caused another potential disaster if it had struck the line.

“[The tree] missed it by a few inches,’’ he said.

Syku said he was shocked that the tree even fell.

“About six years ago, that tree was trimmed, not by us, but by next door. The tree looks very healthy from the outside. Nobody could say in a million years it was going to fall down,’’ the super said.

“I’ve been here for 10 years, and we never had any trouble with that tree. It looks very healthy.’’

Syku added that Monday’s weather was hardly threatening.

“It was not windy at all. Yesterday was completely peaceful,’’ he said.

The super noted that since the tragedy, “The building department has been here to take photos of the tree.’’

In the case of the Brooklyn fatality, a limb on a London Planetree fell about 40 feet and struck tragic Jared Richardson as he sat on a bench in the Reinaldo Salgado Playground in Bedford-Stuyvesant on Saturday morning. Richardson lived across the street from the park.

A Parks Department rep said the tree was last pruned in June 2020 and inspected a month later and found to be in good condition. It also was inspected after Saturday’s tragedy and still in good general condition, the representative said.

The two tree tragedies are only the latest in a series in the city.

In 2020, it was revealed that a couple whose infant daughter was killed by a falling tree limb in Central Park a decade earlier had won a $13.75 million settlement after suing the city, Central Park Conservancy, the Wildlife Conservation Society, which runs the zoo, and the Tenafly, NJ, company hired by the society to do tree work.

The same year the baby was killed, a Brooklyn man died when a tree limb fell on him in Central Park.

-Additional reporting by Tina Moore