The youngest final two in the show's history each sang three tracks at the Nokia Theater during the first instalment in the two-night season ten finale.
A record-breaking 122.4 million votes were cast following the performances, with more than 730 million votes recorded across the entire season, according to Fox.
American Idol has attracted almost 4.8 billion votes since its debut season back in 2002, which concluded with Kelly Clarkson taking home the crown.
Alaina and McCreery will discover who has won this season on tonight's live two-hour episode, which also promises to feature "special performances, surprise guests, a Top 13 reunion".
U2's Bono and The Edge have already been confirmed to appear alongside cast members from the upcoming musical Spider-Man: Turn Off the Dark, while other performers are rumoured to include Lady GaGa and Beyoncé, although Fox has yet to comment.
American Idol airs from 8/7c on Fox.
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