After Ferrari's Felipe Massa labelled Button's 1m18.861s an "incredible" time, the McLaren driver agreed that the lap had surprised him.
"To be fair I was surprised when I saw the dash, but it doesn't mean anything," Button insisted.
"Some people run less fuel than others, and some people run a lot less fuel than others. Who knows? For a first day it felt good.
"I don't know how I did that time. Last year's car was a great car, but rules have changed. They are small, but there are changes.
"And also the tyres are different, not just in compound but in construction."
Button lost mileage with a fuel pump failure in the morning, but was otherwise encouraged by his day.
"Nothing stands out as a big issue.
"Possibly this morning's could be," he said, "but in terms of the way the car feels there's nothing that stands out as something we need to work on.
"Who knows whether it's a championship-winning car, but the important thing is that it has a nice feeling.
"Lap times mean nothing right now, they don't even at the last test, but at the first race that's when you need to show how quick you are.
"The important thing is the car really does relate to the simulator. That's something that is good."
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