MILWAUKEE — For the second straight game in the NLDS, the Chicago Cubs got off to a dream start in the top of the first inning.

And for the second straight game, things took a gut-wrenching turn for the Cubs and their fans in the bottom of the first inning.

The NLDS is at a breaking point now, as the Cubs fell 7-3 and suddenly trail the Milwaukee Brewers 2-0 in this best-of-five series.

Both losses so far have turned on a sequence in the bottom of the first inning.

On Saturday, it was an error and a subsequent rally by the Brewers to plate six runs after Michael Busch led the game — and the series — off with a home run.

In Game 2 on Monday, Seiya Suzuki gave the Cubs a jolt of life with a three-run homer four batters into the game.

As they have seemingly done all season to the Cubs, the Brewers erased that deficit immediately.

Shota Imanaga drew the start for the Cubs despite struggles with the longball lately. He has given up a homer in 10 straight outings — 16 overall in that stretch — including a decisive two-run blast to Manny Machado in Game 2 of the NL Wild Card Series.

This time, it looked like Imanaga was on the verge of a clutch shutdown inning, making the Cubs’ early 3-0 lead hold up. He got off to a dominant start, striking out the first two batters he faced.

But back-to-back two-out singles brought up first baseman Andrew Vaughn, who has suddenly enjoyed a career resurgence in Milwaukee.

Vaughn battled Imanaga to a 3-2 count before crushing a game-tying three-run shot on the seventh pitch of the at-bat.

It was actually not a bad pitch — a sweeper that spun in off the plate:

It was a devastating blow to the Cubs in this series. They had snatched a big lead in a crucial Game 2 and yet the Brewers answered right back.

“With the team putting up three runs, I ruined the game,” Imanaga said through interpreter Edwin Stanberry. “There’s a lot of frustration within myself.”

The Cubs needed a shutdown inning to try to capture momentum and instead, Imanaga’s home run woes continued and the Brewers sucked the life out of Cubs fans.

“We got two outs there — you get an out and there’s no question it’s a different game,” Cubs manager Craig Counsell said. “That’s what three-run homers do. It changes the game dramatically.

” … He did get off to a good start. There was some swing-and-miss the first two innings for sure that was good to see. But the long ball and the rate he’s giving it up right now with runners on base, that’s going to be hard to overcome.”

Two innings later, Milwaukee catcher William Contreras took Imanaga deep to give the Brewers the lead. Left fielder Jackson Chourio later added a three-run shot off Daniel Palencia to send American Family Field into a frenzy — and put the Cubs’ hopes on life support.

The series moves to Chicago for Game 3 on Wednesday evening. With their backs against the wall, the Cubs now have to win three straight games if they want to keep playing into October.