Triple Crown contender Christian Yelich and the Milwaukee Brewers at Wrigley Field. Nolan Arenado and the big-hitting Colorado Rockies at Dodger Stadium.

October baseball is about to begin with Game No. 163 on Monday.

Two tiebreakers on the same day for the first time in major league history. Both to divvy up divisions, too.

“It’s interesting that baseball is such a perfect game in some ways that it takes 162 to not decide anything,” Chicago Cubs manager Joe Maddon said.

Kris Bryant, Anthony Rizzo and the Cubs begin the doubleheader drama when they host Milwaukee for the NL Central crown. Later in the afternoon, Dodgers rookie Walker Buehler is set to start when Los Angeles faces Colorado for the NL West title.

There’s a catch, though.

All four teams are already assured playoff spots, either as a division champ or wild card. The Dodgers, Rockies, Brewers and Cubs knew that going into Sunday, the final scheduled day of the regular season, and then romped by a combined score of 48-5.

So, it means this: These matchups are not one-and-done.

Never before has a team that lost a tiebreaker got to keep playing. Bucky Dent’s pop-fly homer finished off the Red Sox in 1978. Matt Holliday slid home — and maybe touched the plate — to eliminate San Diego in 2007. David Price pitched Tampa Bay past Texas in 2013 to win the most recent matchup.

This time, they’re all in. But, there’s still a big difference between winning the division or going into the postseason as a wild card.

The Brewers-Cubs winner gets to start the best-of-five Division Series at home on Thursday against the wild card winner. The Colorado-Los Angeles winner also begins the NLDS at home versus Atlanta.

“It’s a game where obviously it’s not do-or-die, but I think that it’s a must-win game,” Dodgers manager Dave Roberts said after a 15-0 rout in San Francisco.

“To win the game and get a couple days off, to host the Division Series is very important,” he said.

The tiebreaker losers play Tuesday at the NL Central’s site to determine who advances.

The AL playoff scenario is set.

Oakland and the Yankees hold the wild-card game Wednesday night in New York and the winner goes to Boston on Friday. Defending World Series champion Houston opens the ALDS against Cleveland.

Arenado homered twice and Charlie Blackmon hit for the cycle as the Rockies roughed up Washington 12-0. The Nationals decided to skip ace Max Scherzer and instead start Erick Fedde.

Colorado, which has never won a division title since beginning play in 1993, will try to deny the defending NL champion Dodgers their sixth straight West title. German Marquez, who struck out eight straight batters to begin his last start, takes on Manny Machado, Justin Turner and their Los Angeles teammates.

“It’s two of the best young starting pitchers in the big leagues pitching in this type of game,” Rockies manager Bud Black said. “It’s great.”

The Dodgers topped Colorado 12-7 in the season series. The Cubs are 11-8 against Milwaukee.

At Wrigley, lefty Jose Quintana is set to start for the Cubs. He’s 6-2 with a 1.60 ERA in 10 starts against Milwaukee. The Brewers haven’t announced their starter.

The Cubs put themselves in this position with a 10-5 win at home over St. Louis. Milwaukee won its seventh in a row, trouncing Detroit 11-0.

“Expecting it to be fairly hostile tomorrow,” Yelich said. “Any time we play the Cubs, it’s a hostile environment, especially in Wrigley Field. It’s going to be a lot of fun. We’re looking forward to it.”

Yelich is hitting .323 in his first season with Milwaukee, sealing the first batting title in Brewers’ history. He has 109 RBIs, tied with Arenado and two behind Javier Baez of the Cubs. Yelich’s 36 home runs are even with Colorado’s Trevor Story and St. Louis’ Matt Carpenter, one behind Arenado.

Since the tiebreakers are considered game No. 163 of the regular season, all stats Monday will count, giving Yelich a chance to win the first NL Triple Crown since Joe Medwick in 1937.