skirmish; preliminary match; prefinals
Used in sports and games for a match or round before the finals; also used in military contexts for a small-scale engagement.
He showed brilliant play in the prefinals of the semifinals.
A skirmish between the two armies began.
小競り合い is a more common word for a skirmish or minor clash, often used in non-sports contexts. 前衛戦 is more formal and can be used in both military and game settings.
Compound of 前衛 (vanguard, advance guard) and 戦 (battle, match). The meaning extends from a military skirmish to a preliminary match in games.