The legendary talent spotter and producer admits there were no signs his former protege was using drugs again and he was focused on making her give up another vice - cigarettes.
Davis tells Playboy magazine, "We were trying to get her to stop smoking... It was a serious problem because it affected her singing. She would say, 'I kicked the drug habit, but this is more difficult.'
"She had cut back on cigarettes, but the week she died she came to my bungalow and said, 'I understand I can't just cut back. I've got to stop. I promise I will'."
Houston, who suffered an overdose and drowned in a bathtub in her suite at the Beverly Hilton hotel, had a much bigger problem - she was still taking drugs after many attempts to kick her killer habit.
Davis admits he was oblivious to her problems because Houston was "coherent" and her "spirits were great" - but he'd been fooled by the star before.
He adds, "I only became aware of any problem in maybe the mid-1990s... I didn't know she was in trouble. I didn't know how bad her problem was. It was after a Michael Jackson concert at Madison Square Garden in the late 1990s when I knew (for sure). She showed up ghastly thin. I met with her.
"We talked about the drugs she was using... I tried to help her. I tried to work with the family to help her. She did better for a long time, at least as far as I knew."