opening act; curtain raiser
Refers to the performance or act that comes before the main event, especially in traditional Japanese entertainment like rakugo or variety shows.
opening performer; warm-up act
Refers to the person or group performing the opening act; can also be used figuratively for someone playing a subordinate or preliminary role.
After the opening rakugo performance, it's finally time for the headliner.
He's still an opening performer, but he has the talent to become a headliner someday.