There's a reason we so often hear about head-to-head match-ups during the home stretch of the pennant race. A team can pick up a full game with a three-game series win, and as many as three games with a three-game series sweep. In 2011, the Cardinals swept the wild-card leading Atlanta Braves, turning a 7.5-game wild-card standings deficit into 4.5. And of course we all know how that September (and October) ended.
This brings us to last weekend's series in Milwaukee. The Cardinals (with their -2 run differential and all) led the then-second-place Brewers by three games when St. Louis arrived at Miller Park to start the four-game set. According to the Baseball Prospectus Playoff Odds Report at the time, the Cardinals stood an 81.3% chance of winning the Central, a 9.4% chance of earning a wild-card berth, and a 90.7% overall chance of qualifying for the postseason.
Despite the Cardinals' healthy division odds, the Brew Crew had a chance to make up series ground in the division race (if not overtake the Redbirds). Instead, the Cards took three of four from the Brewers and stiff-armed Milwaukee into third place, and a full five games back in the standings.
On Monday morning after defeating the Brewers, the Cardinals' odds at winning the division had shot up to 92.6% and their overall postseason chances are at 97.5%. Meanwhile, Baseball Prospectus put the Brewers' odds of winning the Central at 2.7% (to the Pirates' 4.7%). Milwaukee stood a 29.4% chance of making the postseason on Monday morning, before losing to the Marlins.
The Cardinals and Brewers have three head-to-head games remaining. Pittsburgh and St. Louis don't play each other again. It's hard to see how either the Buccos or the Brew Crew can make up enough ground to catch the Cards in the Central with so few opportunities to take a big bite out of the division lead.
Head-to-head match-ups are important. They are one of the reasons the unbalanced schedule is so much fun. The Cardinals are currently 11-8 vs. Pittsburgh and 10-6 vs. Milwaukee. Other factors played a part in the Cardinals securing a healthy September lead in the standings, but their head-to-head performance against their primary division competition has been perhaps the largest.