first round vs preliminary

first round

preliminary

adj

  • In preparation for the main matter; initial, introductory, preparatory. 

noun

  • Any of a series of sports events that determine the finalists 

  • A preparation for a main matter; an introduction. 

  • A relatively minor contest that precedes a major one, especially in boxing 

How often have the words first round and preliminary occurred in a corpus of books? (source: Google Ngram Viewer )