Bolt qualified for the semi-finals on Saturday morning without running under 10 seconds but Johnson is adamant that the Jamaican will improve as the competition continues.
"The first thing that stood out was a different Usain Bolt. Not too much of the antics, looking more serious and we shall see how it transpires in the final," the American told BBC Sport. "He never really ran in the race, there was no drive.
"He is extremely relaxed and there is a lot more to come from him. I wonder if he is trying to cover something up, we have not learnt much from this but we will in the next. He is in a good position to defend his title though."