Billy Crystal: Robin Williams tribute was "the hardest thing I've ever had to do"

Billy Crystal says appearing in a special Primetime Emmys tribute for his late friend Robin Williams was the most difficult thing he's ever had to do.

Crystal was a guest on NBC's "Today" show Tuesday to promote the paperback release of his book -- "Still Foolin' 'Em: Where I've Been, Where I'm Going, and Where the Hell Are My Keys?" -- when he was asked by host Matt Lauer about the Emmys memorial.

"It had to be a really tough assignment?" Lauer wondered.

"Yeah, I think it's the hardest thing I've ever had to do," Crystal replied. "It was only two weeks to the day of the Emmys that everything had happened."

"He was my closest friend. And to do it with some humor in only a short period of time that I had to do it, was difficult," Crystal added.

Williams was found dead in his Northern California home on Aug. 11 after committing suicide. He was 63.

Crystal went on to attend the 66th Primetime Emmy Awards on Aug. 25, where he delivered the emotional send-off to his longtime pal.

"It is very hard to talk about him in the past because he was so present in our lives," Crystal said at the awards show. "He made us laugh. Hard...He made us laugh big time...The brilliance was astounding...He could be funny anywhere."

During his "Today" show spot, Crystal was also asked if he knew about Williams' ongoing struggles with clinical depression, to which he answered yes.

"We talked three, four times a day sometimes," Crystal revealed.

The funnyman also spoke of Williams' knack at stand-up comedy.

Crystal said of his talents, "I've never seen anything like him and I don't think we ever will."