The Kiss singer/songwriter had grown close to Madonna during the 1980s as they both shot to superstardom, collaborating on two tracks for the Queen of Pop's 1989 release, Like a Prayer.

Following the singer's untimely death in April (16), Madonna shared a touching message on her Instagram, and it appears she's still struggling to come to terms with his passing.

On Thursday, Madonna shared a picture of the singer in his prime, altering the lyrics of Nothing Compares 2 U, the track he wrote for Sinead O'Connor, to mark the time since his death.

"Its been 7 hours and 13 days since you took your love away..." Madonna wrote. "All the flowers you planted died when you went away... Nothing Compares 2 U." She completed the post with a crown emoji.

After Prince's death, Madonna's manager Guy Oseary revealed that he had wanted the pop icons to hit the road together for a show-stopping tour.

"I called him a few years back with the idea of @Madonna and Prince going on tour together," Oseary wrote in a lengthy Instagram post. "I pitched it to Madonna and within a second she said, 'I like it, we can call it the Royalty Tour... The Queen and The Prince'... I love the way she thinks..."

However, Prince wasn't so sure fans would be able to handle such a huge trek.

"When I told Prince the idea he said: 'the world isn't ready for this, it's too big,'" Oseary recalled.

Prince was found unresponsive at his Paisley Park home on 21 April (16). An autopsy was completed the next day, but the cause of death is still pending further test results.

The U.S. Drug Enforcement Agency and the U.S. Attorney's Office have now been drafted in to help Carver County Sheriffs determine the cause of the Purple Rain singer's death.

Meanwhile, Beatles legend Sir Paul McCartney also paid tribute to the star as he performed at the Target Center in Prince's hometown of Minneapolis on Wednesday night (04May16).

Remembering the "the late, great Prince", Paul told the crowd: "Minneapolis, Prince. Prince, Minneapolis. It goes together."

"Here’s to the man. Loved that guy. Thank you, Prince, for writing so many beautiful songs, so much music," he added later on in his show.