The Giants and Dodgers open a huge series starting tonight at Oracle Park that will play a key role in determining what October looks like for these two teams. The three-game set is the final meeting between the two top NL West clubs in the regular season, who enter with identical 85-49 records. It isn’t just a playoff implication-laden series, it’s one unlike any other we’ve seen since MLB started using divisions in 1969, pitting two teams this good in the same division against each other this late into the season.

Tonight’s matchup in San Francisco is the MLB.TV Free Game of the Day, giving fans throughout the country a chance to tune in to September’s closest postseason race so far. (Blackout restrictions apply, although live audio still will be available through MLB.TV in local markets.) This is what the pursuit of October is all about.

Here’s a look at why the weekend series by the Bay will be historic, with help from the Elias Sports Bureau.

Two teams this good, this late in the season

The Giants’ surprise ascension to the pantheon of baseball’s elite teams in 2021 is well-documented at this point, but let’s consider it through the lens of FanGraphs’ playoff odds. Entering Opening Day, the Giants had a 5.7% chance to make the postseason. The Dodgers were at 98.4%, and the Padres were at 92.3%. Instead, the Giants and Dodgers are now at 100% (they haven’t technically clinched yet, but this happens with projection systems), and the Padres have been below 50% since Aug. 16.

Now, these two teams meet for a series in which they enter with identical .634 winning percentages, tied with each other for best in the Majors. This is just the second season in the divisional era when two teams will meet, at least 130 games into a season, each with winning percentages of .630 or better.

The Yankees and Dodgers did it on Aug. 23-25, 2019, at Dodger Stadium. But here’s the thing: the Yankees and Dodgers aren’t in the same league, let alone the same division. That means that this meeting between the Dodgers and Giants is the first of its kind between two teams in the same division (again, divisions are since ‘69). Talk about a historic rivalry moment.

In this historic rivalry

We know no two teams have met under these circumstances as division rivals. But of course, the Giants and Dodgers have been playing each other far longer than that, dating to the early years of the National League. That was before there were divisions, but they certainly were rivals.

In the entire span of this storied rivalry, going back to their first meeting as MLB clubs on May 3, 1890, this is just the second season when these two teams will meet, 130 or more games into a season, each with .630 or better winning percentages. The other time was Sept. 3-6, 1962. And this season’s series starts on? Sept. 3. Because baseball.

That Sept. 1962 series, and the rest of that ninth month of the year, led to what happened in the beginning of October, 1962 -- a tiebreaker series for the NL pennant between the Giants and Dodgers. That’s right, the only other time these two teams met this late in a season, both being this good, they eventually had to go to a tiebreaker series.

The Giants prevailed in the ‘62 tiebreaker, with a four-run ninth inning in the decisive third game to book a trip to the World Series, where they lost to the Yankees in seven games. This weekend’s winner won’t be booking that World Series ticket quite yet, but that team will find itself with a sole division lead.

Evenly matched so far

There are ties and equal numbers all over the place with these two teams entering Friday. To start with, they’re tied for the NL West lead. It’s the second season in which they’ve been tied for first place in the same division in September, since divisions began in ‘69. The other year was 1997, with the most recent date being through the games of Sept. 18. The Giants went on to win the division that year, losing in the NL Division Series to the eventual World Series champion Marlins, while the Dodgers missed the playoffs.

In all the years they’ve been in the same division, since that first year in ‘69, these are the only two times that has happened in September. Pretty wild.

And these teams aren’t just tied in overall record -- they’ve also evenly split their first 16 meetings so far, eight wins apiece. And to add further equal numbers, each team has scored exactly 68 runs so far. It’s that close.

What’s next

This would be just the third time both the Giants and Dodgers make the playoffs in the same year -- something that, of course, was not possible until the introduction of the Wild Card with the 1995 postseason. Both teams made it in both 2014 and 2016, but they didn’t face each other in October. Could that end up in the cards? In recent years, we’ve seen some other historic rivalries play out in the postseason -- the Red Sox and Yankees in the 2018 ALDS and the Cubs and Cardinals in the 2015 NLDS, which was the first postseason meeting between those two NL clubs.

And as for both teams being on a 102-win pace? Since the Wild Card was first employed in the 1995 postseason, we’ve seen two divisions sport multiple 100-plus game winners: the 2018 AL East, with the Red Sox (108) and Yankees (100), and the 2001 AL West, with the Mariners (116) and A’s (102).

There are more instances before the Wild Card of teams winning 100-plus games and finishing second, including a relatively recent one that stands out to Giants fans, when they won 103 games but missed out on the postseason in 1993, when the NL West’s Braves won 104.

A historic series, in many ways, gets underway tonight in San Francisco.