Ben Grimm was transformed into The Thing at the same time when his fellow members of the Fantastic Four received their powers.
When the Sentry first saw the orange, rocky figure of The Thing after hearing word of a gang hiring a villain named Rockslide to take him out, he automatically assumed that The Thing was Rockslide. In the ensuing fight, Sentry was able to defeat The Thing, and soon afterwards he realized that he had made a mistake when the real Rockslide showed up and quickly surrendered after seeing how he was able to defeat The Thing.
The Thing would get an opportunity for redemption for his loss to the Sentry when he got into a fight with the Juggernaut in a bar. The ruckus caused the Sentry to show up, and the three powerhouses went at it triple threat style. The fight ultimately ended in a stalemate, with the most damage being done to the building (the only part of the structure left standing was the actual bar itself with the bartender standing wide-eyed behind it).