A Google engineer enjoying a quiet walk in Central Park Wednesday was hit on the head by a large branch that snapped from a tree — a freak accident that left the father of two in a coma.

Sasha Blair-Goldensohn, 33, was alone on a footpath between Central Park West and West Drive near 63rd Street at 8:15 a.m. when the rotting branch suddenly broke and fell on him from 20 feet above.

He was instantly knocked unconscious.

His wife, Rebecca Min, and his family anxiously waited for news at New York Hospital, where he was in critical condition.

An uncle, Marty Goldensohn, 62, said doctors are hopeful.

"He's progressing. He's responding to touch and command, but it'll take time," he said.

He described his nephew as "a brilliant guy with IQ to spare."

Blair-Goldensohn, the father of two girls, was hired by Google in 2007 after earning a Ph.D. in computer science from Columbia.

"This is just horrifying," said his graduate adviser, Professor Kathy McKeown.

When he did not show up for work at Google's Manhattan offices Wednesday, his colleagues became concerned.

"Everyone is really upset and shocked," one co-worker said.

Parks officials said the limb on the pin-oak tree was more than four inches in diameter.

The branch was dead wood, rotted out in the interior, and the fall had nothing to do with the bad weather, Parks sources said.

The tree was inspected after the incident, and was found to be sound.

But the branch "was rotted out and it was liable to fall, and, unfortunately, it did," a Parks employee said.

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