Bono was ''terrified'' of Alicia Keys when he first met her, as the singer had him ''shaking'' in his boots.
He said: "I was terrified the moment I met her. I was shaking in my boots. I was very moved by her singing of course, but what was interesting was the hard questions afterwards, and I think it's those hard questions that she asks that lead her.
"She has a lioness energy."
The Irish rocker explained Alicia - who has a 13-month-old son Egypt with husband Swizz Beatz - has both The Head and the Heart to successfully front her Keep a Child Alive charity, which provides assistance to those affected by HIV and AIDS in Africa and India.
Speaking at the premiere of new documentary 'Keep a Child Alive with Alicia Keys', Bono added: "She won't let other mothers suffer.
"Everyone's got heart, but actually you have to have the head for this. You have to be tough and strategic, you have to be demanding, and the money has to be spent well. If the money isn't spent well, people get annoyed and so all these things take a certain intellectual rigor."
'Keep a Child Alive with Alicia Keys' documents a visit the singer made to South Africa during last year's soccer World Cup and airs on Showtime tomorrow (01.12.11), which is World AIDS Day.