The musician and coach on NBC's The Voice alongside Shelton appeared on the Today Show Thursday to discuss his colleagues' romance as well the upcoming season of the singing competition.
"It's poppin' off," Williams said. "It's so beautiful, because being there, I watched both of them go through a lot, you know. You hate to see your friends go through something so heavy, and it's kind of like a miracle, man -- just watching that."
He said he had no idea the couple would get serious after their respective divorces last year. "And then Blake sends me a text message, and I'm like, 'Whoa!'" he added.
On Oscar night last Sunday, Shelton and Stefani made their first red carpet debut as a couple. The 46-year-old "Make Me Like You" singer and 39-year-old country star attended the Vanity Fair Oscar Party in Beverly Hills together.
The couple confirmed their dating relationship in November.