It was a goal as slick as the rain-soaked turf at Veterans Field in East Northport, a goal so slick and so subtle that the scorer was not correctly identified until an hour after the game.

“I got a text from my assistant coach that he got a text from the player,” Northport coach Aija Gipp said Thursday night.

The message was that Cybele Laisney had informed one of the coaches that she had headed home a free kick by Stephanie Rapp for the only goal of No. 1 Northport’s 1-0 victory over Sachem East in a Suffolk AA girls soccer quarterfinal.

It was a goal, scored in the 18th minute, that officials, coaches and media members had initially credited to Rapp, who after the game said she thought, “The ball must have gone between the goalie’s legs.”

But Rapp, the team’s top defensive player, acknowledged she had aimed her free kick from about 30 yards out to go to Laisney. “She is really good in the air,” Rapp said.

Gipp called the play “a set piece. We use Stephanie on free kicks because she is so accurate with them. And Cybele just flicked it with her head and re-directed it on goal. She’s our shortest player but she’s our best player in the air. Our defense has created a lot of opportunities for our offense this year so it was fitting that a defender set up the only goal.”

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Northport’s best defense Thursday was its offense, which dominated play and kept the ball in Sachem East’s end for most of the game. “A couple of inches either way and it could’ve been 3-0 or 4-0,” Gipp said. “We kept our composure and created a lot of scoring opportunities, and that alleviated pressure on our defense.”

Northport (16-0), which will host Ward Melville in a semifinal at 2:30 p.m. Saturday, was especially relentless in the second half, when Shannon Hearty was forced to make five of her eight saves for Sachem East (9-4-3).

Hearty made a brilliant one-handed deflection to rob Emily Cerrito in the 58th minute. Leah Dentale buzzed the net frequently for Northport, and one of her shots bounced just wide in the 69th minute.

Rapp and her teammates made sure those near-misses weren’t costly when they turned away a flurry by Sachem East in the final minutes. “They did pressure us at the end, but everyone came back to help the keeper,” Rapp said.