The Cubs (87-61) jumped on the Cardinals (92-56) early and held on amidst a pressure-packed ninth inning en route to a 5-4 win over the Redbirds in front of 40,994 at Wrigley Field.
Cubs closer Hector Rondon came out to pitch the ninth but was immediately ejected after hitting leadoff batter Greg Garcia with a pitch.
Both benches had been warned the previous inning after Fernando Rodney hit Kolten Wong with a pitch following Friday's beanball incident. Joe Maddon was also ejected in the ninth.
Zac Rosscup came in and promptly allowed a two-run homer to Matt Carpenter and then back-to-back singles to Tommy Pham and Jason Heyward.
Pedro Strop came on and struck out Jhonny Peralta, induced Yadier Molina to fly out to right field (bringing in the Cardinals' fourth run) and then Addison Russell made a ridiculous diving stop to end the game.
"We didn't hold on to it earlier in the year," Maddon said. "That's the difference between then and now. Earlier in the year, we were not able to hold on against these guys late. Right now, we're holding on and that's a good thing.
"I said at the beginning of the year, they were out-experiencing us and right now, we're catching up in that regard. So give them a lot of credit because they always fight 'til the last drop and I do respect that."
After starting out 4-9 against the Cardinals this season, the Cubs have now won four of their last five games against the division leaders.
A big part of the Cubs' success recently is scoring in the first inning, which they've done in each of those five contests, totaling 10 runs in the opening frame.
Kris Bryant's double and Starlin Castro's two-out single helped plate two early runs for the Cubs off Michael Wacha Saturday.
"Our guys came out ready to play today," Maddon said. "It was pretty impressive the way we started that game against a pretty good pitcher."
The Cubs tacked on with back-to-back homers from Jorge Soler and Bryant in the fifth and a pinch-hit RBI single from Tommy La Stella in the sixth.
Bryant's homer was his 25th of the season, tying Billy Williams for club record for a rookie.
Maddon went with a "bullpen day" Saturday, as Travis Wood and Trevor Cahill combined to allow just one run in the first six innings.
Clayton Richard, Justin Grimm and Fernando Rodney shut the Cardinals down to bridge the gap before the ninth-inning drama.
The Cubs reveled in the playoff atmosphere at Wrigley, with almost 41,000 fans screaming and hanging on every pitch right up until Russell's diving stop.
"That [atmosphere] was the best I've ever seen in a baseball game, personally," Bryant said. "It was a lot of fun. Both sides were really competing.
"Addison - I think that's probably the best play I've seen given the circumstances and where we're at in the season. I mean, to sit on the bench for eight innings and then come in and make a play like that, you just can't make these things up."
The Cubs are now only five games back in the National League Central and have won seven of their last nine against the Cardinals and Pittsburgh Pirates.
"We want to win the division," Bryant said. "We're showing emotion and we're ready to fight for it.
"We don't want to settle for the wild-card spot. We want it all."