There was no guarantee on Opening Day 1921 that two teams with such vastly different playing styles — the Deadball Era strategic baseball of the New York Giants vs. the heavy-hitting Lively Ball game of the New York Yankees — would find themselves facing off against one another in the World Series. During the regular season, both teams engaged in nail-bitingly close pennant races even as they remained unhappily tethered to one another at same home ballpark: the Polo Grounds.
In spite of the Yankees’ strong pitching staff and a batting lineup that included the Sultan of Swat, Babe Ruth, it was not clear if the American League Yankees would be able to grasp the pennant away from the defending World Series champion Cleveland Indians, with their talented player-manager Tris Speaker. In mid-September, Cleveland regained first place in the American League standings and a crushing Yankees defeat by the Tigers a few days later suggested the Yankees might be on a permanent trajectory downward.
As of late August, the National League Giants remained behind the first-place Pittsburgh Pirates, who — with a hard fought win over the Boston Braves on August 22 — increased their lead. These two pennant-contending teams entered into a five-game series on August 24 with the Pirates ahead by 7½ games. On August 27, the Giants emerged from this head-to-head battle having made a major step forward with a five-game sweep.
On September 9, the Giants finally managed to move into first place. The Yankees took until October 1 to clinch the American League pennant. Qualifying for the World Series had remained a contested likelihood for both teams almost until the fall classic was scheduled to start. In the end, they did play one another — and by doing so, they made history: the 1921 World Series was the first modern World Series to be played all in one stadium, and the first consisting only of teams based in New York.
— Sharon Hamilton