noun
first round; first game
Used in sports and competitions for the initial round of a tournament or match.
彼は一回戦で負けてしまった。
He lost in the first round.
テニスの一回戦は明日行われる。
The first round of the tennis tournament will be held tomorrow.
準決勝 means 'semifinal', a later round in a tournament.
Compound of 一回 (one time, first) and 戦 (match, battle).