In colloquial usage, the English would commonly be 'act', not singers (as in 'opening act', 'support act').
'opening act' = 'babak pembuka'.
That is not necessarily about singing.
It could be (for example) an opening act from a ballet performance.
I guess you could say "Opening Singing Act"...I'm not sure if that's better than "Opening singers"
That is a totally incorrect sentence in English (Australia).
"They are the opening singers"? I disagree, it's fine.