Hales's fabulous knock, laced with 17 boundaries, blew away the Lankan bowlers, who also struggled to grip the ball due to heavy dew.
"Centuries in this format don't come often. I've had a couple of 90s and it's good to get over the line. It's great to get on the board in the tournament. Without a shadow of a doubt it's the best I've ever played," Hales said after England's six-wicket win which keeps them in the hunt for a semifinal place.
With Eoin Moragn he raised a 152-run stand for the third wicket.
England skipper Stuart Broad could not stop smiling and termed Hales' knock as one of the best he has seen someone playing for his country.
"That's a very special knock and one of the best I've seen in an England shirt. World events are a big and that kind of win will give our dressing room a lot of belief," he said.
"I feel for Sri Lanka's bowlers with the dew. We changed about four or five balls in the innings. We've been fielding with wet balls at training, we should probably make it wet bars of soap I think," he said.
