10:20 PM PT -- Roberta's rep tells us ... the singer's still in the hospital, and doctors are still running tests to determine why she fell ill during the Apollo show. The rep adds, Roberta had a stroke in 2016, and doctors want to make sure this episode is not related to the stroke.

As for her symptoms ... we're told she felt extremely dizzy as she was supposed to walk out onstage to receive an award. Due to Roberta's age and the previous stroke ... paramedics were called immediately.

"Killing Me Softly" singer Roberta Flack's performance at the Apollo Theater was cut short Friday night as she was taken by ambulance to a hospital ... TMZ has learned.

Roberta was performing a charity concert for the Jazz Foundation of America when she fell ill. FDNY tells us the ambulance arrived around 8:50 PM EST, and she was taken to a hospital in Harlem.

We're told the 81-year-old singer Roberta was able to leave the stage under her own power ... before someone called for the ambulance. FDNY says they were responding to a call for a "sick person."

We've got a call in to her rep for an update on her condition.