“There is no more reason why the champions should not have been victorious in the aftermath than in the curtain-raiser, but for stupid work on their own part, after they had practically clinched a victory in the ninth inning.” “Won and Lost a Game. Easy Victory, Humiliating Defeat.” The New Era (Lancaster, PA), June 15, 1898, p8.
NOTE: There are earlier uses of the term for the opening match of a boxing card, as well as one where a boxing match was the curtain raiser before a baseball game in Canada.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1909. “[Frank] Chance ran in and covered the plate when they were running [Honus] Wagner to death in the first inning of the curtain raiser.” (Chicago Tribune, May 3; Peter Morris).