The Chicago Cubs learned Saturday afternoon how fleeting momentum can be for teams on the outside looking in on the National League wild-card race.

On Sunday night, the Cubs will face a team — the St. Louis Cardinals — that is in a much more favorable spot in the wild-card standings.

The Cubs lead the key four-game set between the longtime NL Central rivals 2-1 heading into the finale in Chicago.

Right-hander Miles Mikolas (8-8, 4.99 ERA) is slated to start for the Cardinals against left-hander Justin Steele (2-5, 3.38).

The Cardinals prevailed Saturday when Lars Nootbaar lofted the tie-breaking sacrifice fly in the ninth inning of a 5-4 victory.

Tommy Pham led off the ninth with a triple before scoring on the flyout by Nootbaar for the Cardinals, who overcame an early 4-1 deficit. With the win, they are two games behind the Arizona Diamondbacks, who currently hold the third and final wild-card spot in the NL.

Kyle Gibson and relievers JoJo Romero and Ryan Helsley combined to blank the Cubs on two hits over the final seven innings Saturday, but the effective pitching was in danger of being wasted before a pair of fielding miscues by Chicago led to the Cardinals’ game-tying, three-run rally in the eighth inning.

With two on and two outs, Cubs third baseman Isaac Paredes threw the ball away on Brendan Donovan’s grounder, which allowed Alec Burleson to score from second. Two pitches later, Nolan Arenado hit a seemingly routine pop-up into shallow center, but second baseman Nico Hoerner was called off by rookie center fielder Pete Crow-Armstrong. The ball glanced off the glove of a sliding Crow-Armstrong as two runs scored.

“You get a run out of a throwing mistake,” Gibson said. “And then I told Nolan when he walked in (to the dugout), ‘Sometimes you’ve just got to put the ball in play and take advantage of little bloops.'”

The little bloops helped to end a three-game winning streak for the Cubs, who missed a chance to pull within three games of .500 for the first time since July 12. Chicago is six games behind the Diamondbacks in the wild-card standings — with five teams between them.

“Pete just got a little over-aggressive,” Cubs manager Craig Counsell said. “It’s right to call him off, but not if we’re making a sliding play.”

The loss came in what likely will be one of the Cubs’ final chances to make up ground in a head-to-head fashion against fellow fringe wild-card contenders. After Sunday, Chicago plays its next 14 games against American League foes.

Once the series with the Cardinals concludes, the Cubs’ six remaining games against the Pittsburgh Pirates — scheduled for Aug. 26-28 and Sept. 2-4 — are their last against teams in the wild-card race.

Mikolas didn’t factor into the decision in his most recent start July 28, when he allowed three runs over six innings in the Cardinals’ 4-3 win over the Washington Nationals.

He is 6-5 with a 2.97 ERA in 22 career games (20 starts) against the Cubs, including 1-1 with a 5.09 ERA in three starts this season.

Steele took the loss on Tuesday after giving up five runs over five innings as the Cubs fell 6-3 to the Cincinnati Reds. He is 4-1 with a 3.47 ERA in six starts against the Cardinals.