Thomas Greiss tracked Jonathan Huberdeau as he prepared for his attack midway through the second period. The goalie’s stick had sailed away to the right of him, and Thomas Hickey, the closest teammate, wasn’t aware that Greiss was more or less defenseless. And so Greiss did the thing that he’s gotten very good at this season: He improvised. He shifted his blocker to where his stick would be. He made the save.

This has been Greiss in a nutshell — a so-called second- string goalie who spent the better portion of a month in the shadow of Jaroslav Halak. But for the second game in a row, when things went south or when his teammates didn’t produce or protect enough, Greiss gave them a shot. “The series has been going back and forth the whole time,’’ he said, “and you try to make the stops when you can, and that’s pretty much it.”

On Friday night, that meant making a whopping 47 saves en route to a win in the second overtime. In a no less formidable performance Sunday night, it meant stopping Florida when the Islanders fell completely flat in the second period and keeping the Panthers at bay in a 2-1 double-overtime win.

“He played great,” coach Jack Capuano said. “You need goaltending and he came up big for us and there was no doubt, when it was 1-0, we didn’t have our legs for whatever reason . . . and Thomas made some great saves and kept us in.”

Entering Sunday night, Greiss had a .938 save percentage in the playoffs, and he made 41 saves in Game 6. For a while, he was the only reason the Islanders had a chance to win a series for the first time since 1993.

Greiss found himself playing the part of human equalizer. When the Islanders spent almost all of the second period skating circles in their own zone, there he was, blocking shot after shot. With or without equipment. He made 12 saves then and added 13 more in the third. “He’s let us get our game going,” said John Tavares, who scored the equalizer with 53.2 seconds left in regulation and then the winner. “Once we get our game going, we’re able to finally get him some support.”

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Thirty seconds after Greiss’ stick-less stop on Huberdeau, Jussi Jokinen shook Travis Hamonic and went in on the goalie alone — and that point-blank shot also was stopped. Then came a mad scramble in front of the net with less than five minutes left in the second with Greiss facedown on the ice after making a stop and Jaromir Jagr, Dmitry Kulikov and Aleksander Barkov circling. Jagr failed to sweep it in and Greiss was able to shove Kulikov out of the way.

Said Greiss, “I think it was our best game of the series and both games played very tight and it was an exciting game until the end.” All credit to him for keeping it that way.