MILWAUKEE — The vibes were immaculate for the Chicago Cubs to begin the NLDS after first baseman Michael Busch led the game off with a home run.

But it turned sour very quickly for the Cubs — and the entire game may have changed on one play.

The Milwaukee Brewers rattled off three straight doubles to begin the bottom of the first inning, quickly capturing the lead. After a groundout and a walk, Brewers right fielder Sal Frelick hit a slow-moving groundball towards Cubs second baseman Nico Hoerner.

The ball was hit at only 49.5 mph, but it slipped under Hoerner’s glove as he charged in.

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Brewers catcher William Contreras came around to score on the play, but that wasn’t the end of the damage.

Cubs starter Matthew Boyd struck out the next batter, but couldn’t put away Milwaukee’s No. 8 hitter, Blake Perkins, who singled on the 11th pitch of the at-bat to drive home two runs — and drove a dagger into the Cubs’ Game 1 hopes.

That hit bounced Boyd from the game, forcing Cubs manager Craig Counsell to go to his bullpen in the first inning.

There’s no guarantee Boyd and the Cubs would have escaped the jam, but if Hoerner had made that play, it would have been two outs with runners on second and third. And most importantly — the Brewers would have only held a 2-1 lead.

Four unearned runs followed, and the momentum swung decisively to the Milwaukee dugout.

It was an uncharacteristic mistake from Hoerner, who made only four errors in the regular season and should be a Gold Glove finalist in the NL this year.

It was also a stark contrast from the NL Wild Card Series, in which the Cubs’ defense was the deciding factor — especially in the middle of the field with Hoerner, shortstop Dansby Swanson and center fielder Pete Crow-Armstrong.

That trio made highlight-reel play after highlight-reel play against the Padres.

But it was defense that did the Cubs in Saturday, setting in motion a record-setting start for the Brewers. The hosts went on to win 9-3 and take a 1-0 series lead.

The NLDS will take a break Sunday before resuming Monday night in Milwaukee for Game 2.

The series shifts to Wrigley Field for Game 3 on Wednesday.