WASHINGTON — Almost immediately, the comparisons began, as soon as the dunk hit social media. Zion Williamson’s eye-popping alley-oop slam was being compared to Grant Hill’s jam in the 1991 national championship game.

“The ball was so high. [Bill] Raftery thought the ball was going out of bounds,” Hill, ironically, calling the game for CBS told The Post after top-seeded Duke’s thrilling 75-73 win over No. 4 Virginia Tech in an East Region semifinal Friday night. “He’s spectacular. I don’t know if anyone [has ever] jumped that high. I know I didn’t jump that high.”

It came at a critical juncture, capping a 7-0 spurt that gave Duke a 58-52 lead with 10:31 remaining. After a Tre Jones steal, the freshman tossed a lob to the on-rushing Williamson. It seemed too high, like a momentum-killing turnover. But Williamson kept rising as part of his astonishing leap, and easily caught the pass before flushing the ball with both hands.

“I know how high Zion can jump, and that’s a momentum play for us.” Jones said. “Being able to put it up there, I know he’s going to go get it.”

Added Williamson: “He threw it up there, so I went and got it.”

Duke coach Mike Krzyzewski declined to compare the dunks, though he raved about both of them.

“Zion can do amazing things. He can actually jump higher than Grant. But on that play that Grant did with Bobby [Hurley], he was, I think, the highest he’s ever jumped,” the Duke coach said. “And when you’re playing with a great point guard, sometimes they lead you and put you in a position where you extend and you pass the limit you already had, because they have such confidence in you.”

Hill said there was one big difference between the two plays: The stakes were higher in his game.

“You know, mine was in the finals,” he said jokingly. “You do it in the finals, which if they’re fortunate enough to get there, I’m sure given the opportunity he will. Until he does that, I’m not impressed.”