The Nets never gave themselves a chance in Monday’s playoff opener right from the start. But Wednesday, it was the endgame that did them in.

After proving they can play with defending-champion Toronto, they still haven’t shown they can beat the defending champs. They had a chance but fumbled it away in a 104-99 loss in the Eastern Conference first-round Game 2.

The Nets — who dug a 33-point first-half hole in the opener — ran out to a 14-point first-quarter lead and held a six-point edge early in the fourth. But they went ice cold offensively, down by just three in the waning seconds before coming up empty on their final possessions.

“We turned it over, gave them too many transition points. Too many mental mistakes in the fourth quarter. We’ve got to be better,” said Garrett Temple, who had a team-high 21 points. “Playoffs, you can’t have as many mental mistakes because they come back to bite you. Every possession counts.”

Like the last two. Up 80-74 after three quarters, the Nets got outscored 30-19 in the fourth. They shot just 35 percent with five turnovers — none more costly than the last, charged to Joe Harris with 8.5 seconds left.

Down 102-96, the Nets halved the lead on a 3 by Timothe Luwawu-Cabarrot (17 points). They forced a shot-clock violation to get the ball back, but Kyle Lowry forced a turnover on a handoff between Temple and Harris (14 points, 15 rebounds).

Norman Powell — who had a dozen of his game-high 24 points in the fourth quarter — went the other way for a breakaway. Temple missed a 3 with 0.4 seconds left, and the Nets find themselves in an 0-2 hole going into Friday’s Game 3 in Orlando.

“You’ve got to embrace it, open up to the challenge of getting one game at a time. Our group will respond,” interim coach Jacque Vaughn said.

“You look at the first game, we didn’t play so well but we still had a chance. This game, we were in it for the whole game except for the fourth,” said Jarrett Allen — who had 14 points, 15 rebounds and five assists. “So the message is: It’s a long series. The series isn’t over, and we can come and take it if we want it.”

It’s not over, but it’s damn close. Of 417 teams that have faced an 0-2 hole entering this year, just 27 have prevailed. At 6.5 percent, advancement isn’t likely.

“We weren’t likely to be the seventh seed once seven of our players didn’t come to the bubble. We weren’t likely to beat the teams we beat down here in the bubble,” Temple said. “We enjoy being underdogs. None of those stats mean anything to us. We’re going to come out and play the game we know how to play and try to win on Friday.”

Now they’re down another player. Harris left the bubble after the game for non-medical reasons.

Still, it was more auspicious for the Nets, who jumped ahead 12-5 on a Temple 3.

Caris LeVert (16 points, 11 assists) drew three defenders and fed Allen for an alley-oop and a 26-12 cushion with 5:39 left in the first.

With the score knotted at 53-all early in the third on a 3 by Lowry (21 points), Brooklyn reeled off 10 unanswered points.

Temple had 15 in the third quarter to keep the Nets ahead, 80-74, going into the fourth. But after the Raptors started switching from a 2-3 to a box-and-one and other defenses, the befuddled Nets allowed a 19-5 run and fell behind by eight on Serge Ibaka’s free throws with 6:21 left.

“They did a good job of switching their defenses up,” said LeVert, who shot 5 of 22. “It seemed like it was a little stagnant.”

Down 102-96, Luwawu-Cabarrot’s 3 halved the lead with 35 seconds to play. The Nets got the stop they wanted, but never got off the 3 they needed.