While Harrison was defeating Rob Calloway in London, Williams suffered a six-round loss to Turkey's Sinan Samil Sam in Berlin.
Harrison told BBC Radio Five Live: "It was a shock but you've seen what happens when you put your neck on the line.
"Danny had been getting a lot of publicity off my back, but he is not more than a domestic fighter.
"I have said it time and time again when he steps up he will be exposed. You saw that last night."
Harrison made it nine wins out of nine when his opponent suffered a broken jaw at the end of the fourth round.
But he was not pleased with his performance. "I would give myself five out of 10," he said.
"The guy's style and hand speed caused me a few problems. It was not vintage Audley Harrison but effective Audley Harrison."
For the first two rounds, Calloway's movement and sneaky right hand troubled Harrison, who appeared reluctant to commit himself to going forward.
But his size and jab began to take over in the third round as Calloway started to tire.
The American had never been stopped before in a career spanning over 50 fights, most of which were contested at light heavyweight and cruiserweight.
Harrison remains unbeaten after nine fights and intends to return to the ring on 29 March at Wembley.