The Four Tops leader Abdul 'Duke' Fakir has been left "truly heartbroken" by the sudden death of The Supremes star and his former fiancee Mary Wilson.

The You Can't Hurry Love hitmaker passed away late on Monday, aged 76, and Fakir reveals her unexpected loss was all the more shocking because they had just spoken over the phone days earlier.

"She was in good spirits, said she was feeling great," he told Billboard of what would become their final conversation. "She was working on some kind of new project, which was normal for her. She was always trying to do something different and new."

The musicians had grown close in the early 1960s after the Four Tops signed to Motown, and he recalls becoming "instant friends" with Wilson.

"Of course, it grew into something a little more than that," he said of their brief 1964 engagement, which ended with the stars deciding to keep to friends instead of lovers - and they remained close confidants until the end.

"We've been lifelong friends, dear friends," he shared. "She was probably one of the sweetest people I ever met - so full of life, so joyful.

"She was such a sweetheart and had so much energy. So this (death) was the last thing I expected. I'm really in a state of shock."