skirmish; preliminary contest; prefinal
Used for a preliminary battle or contest that precedes the main event. In sports and games, it often refers to the semifinal or a match just before the final.
A skirmish between the two armies began.
This match is regarded as a prelude to the final.
準決勝 is the standard word for 'semifinal' in tournaments. 前駆戦 can be used similarly but is less common and carries a nuance of being a preliminary or precursor event.
Compound of 前駆 (forerunner, precursor) and 戦 (battle, match). The exact historical derivation is uncertain, but the word follows standard Sino-Japanese compounding patterns.