For three quarters of the G League Eastern Conference final, the Maine Celtics were cold, sluggish and sloppy – and off when they could least afford to be.

And then everything clicked. And a party broke out at the Portland Expo.

DJ Steward kick-started a dominant fourth quarter and finished with 27 points, JD Davison scored 19, and the Celtics reached the G League finals for the first time in their 15-year history with a 99-77 victory over the Long Island Nets in front of 1,832 delirious fans Sunday night.

Maine (23-13) will face the Western Conference champion Oklahoma City Blue in a best-of-three series that starts Tuesday at the Expo.

“It feels amazing,” Steward said. “This team is very special. We love each other, we’re always hanging out with each other. To win the Eastern Conference finals is very big, but the job’s not finished.”

The fourth-quarter outburst – 3-pointers in bunches, breakaway dunks, full-throated screams and chest thumps – seemed to come out of nowhere after Maine shot 36.8% from the field in the first three quarters, including 30% in the third as an 11-point first-half lead shrank to a one-point advantage.

“The whole game, we couldn’t throw a rock in the ocean,” said Jordan Walsh, who scored 15 points. “We knew, at some point, the tides were going to shift.”

Steward got the run started, driving for a layup and then connecting on back-to-back 3-pointers to make it 69-62 and prompt a Long Island timeout with 10:36 to play.

“I felt like it got us going,” Steward said. “But it’s my teammates. They always come up to me and they tell me to keep going, keep pushing and being aggressive. When I have them telling me that, it gives me the ultimate confidence to just go out there and ball.”

The timeout didn’t stop the Maine onslaught. Neemias Queta (16 points, 19 rebounds) rejected a dunk attempt by David Muoka, then threw down an alley-oop off a feed from Davison (11 assists) to make it 73-64. Walsh then connected on a 3-pointer, and followed it up three possessions later by grabbing a rebound and slamming down a breakaway dunk for an 80-66 lead, pounding his chest in front of the roaring fans.

“It felt like it was the loudest this building’s ever been,” Walsh said. “It wasn’t the most people that’s ever been in the building, but it (was) definitely the loudest.”

The fans rarely had a chance to quiet down. Maine shot 78% (14 for 18) in the fourth, including 6 of 8 from 3-point range.

“We started making shots, and DJ Steward is good at basketball,” Coach Blaine Mueller said with a smile. “So that helps a lot. … It really felt like, for the most part, we had some great looks. We kept generating them, and then really, they started falling.”

It wasn’t just a shooting explosion. Maine’s defense, which kept the Celtics afloat through the dry spell, reached another level in the fourth, holding the Nets to 33% from the field. Long Island shot 38.6% for the game.

“That was probably our best fourth quarter this season,” Steward said. “I just felt like we locked in, collectively, as a group. Defending, getting rebounds, and we forced them to take a lot of tough shots.”

Maine defended effectively without fouling. Long Island shot 40% in the semifinals but still scored 120 points on the strength of 19-for-24 free-throw shooting.

The Celtics, though, limited Long Island to four free throws.

“The free-throw line was a huge, huge, huge, huge piece,” Mueller said. “It’s something we’ve really talked about all year, and we’re frankly not great at it most games.

“Tonight, across the board, the game plan and execution … was the difference in the game.”