"He’s my closest friend, and to do it with some humor and only in a short period of time that I had to do it was difficult," the 66-year-old said Tuesday of the tribute. "It's the hardest thing I've ever had to do."
He compared the experience to being "the spokesman for everybody in front of 17 million people."
At the Emmys, Crystal said of his friend, "It's very hard to talk about him in the past because he was so present in all of our lives. For almost 40 years, he was the brightest star in the comedy galaxy."
Crystal recalled to Lauer that his longtime friendship with Williams really blossomed when they started working with Comic Relief and personally delivering money raised by the telethons. "So it was on those plane rides or on the car rides, being those quiet moments where we weren't just doing comedy, we were just talking as friends."
As for the darker times with Williams, when he was battling depression, Crystal said, "We’d talk three, four times a day, sometimes, but then the riffs were still great. We'd go late into the night like two jazz musicians who just got new horns.”
Williams passed away on Aug. 11, 2014. He was 63.