The singer-actress, 26, and her former hockey-pro husband, 33, "have mutually decided [upon] an amicable separation," a rep for Duff confirms to PEOPLE exclusively.
"They remain best friends and will continue to be in each other's lives. They are dedicated to loving and parenting their amazing son, and ask for privacy at this time."
A source close to Duff, who wed Comrie in August 2010, says, "There was no major incident – no big event between the two of them. They really did drift apart. They have put effort and thought into this decision for a long time. They tried couples therapy, but in the end, they realized at this time, they work better as friends. And they really are great friends."
The couple plan to co-parent 21-month-old son Luca Cruz. "They're still going to go to the park together," adds the source.
The split was not "a rash decision," the source tells PEOPLE. "It's something [Hilary] thought through," the source explains. "She really does care about him and he really does care about her."
The former Disney star spoke effusively about her husband in 2009: "I've never met anyone who can say a bad word about him," she said. "He's generous, caring and sunny. We just laugh our heads off."
Duff told PEOPLE in October that Comrie has since become a wonderful dad as well.
"He's an amazing father," she says. "He's so patient and loving and involved. He's all the things that I knew that he would be."
Mike and I are sitting here,we are so appreciative for all of your kindness& well wishes. Not an easy day but we're getting thru it together— Hilary Duff (@HilaryDuff) January 10, 2014