After twiddling his thumbs on the opening day of pre-season testing on Tuesday due to technical issues with the new McLaren, the 34-year -old was top of the timesheet on day two.
The times, however, are largely irrelevant at present given the steep curve the teams are on when it comes to learning about the new cars in light of a raft of regulation revisions.
One of those has seen the weight limit of car and driver combined rise from 642kg last year to 692kg this season.
But that is arguably not enough for the taller drivers such as Button given the arrival of the new powertrains, in conjunction with the new energy recovery system that have made the cars significantly heavier.
Previously on the limit anyway, Button has been forced to diet further, but has revealed he needs to lose more weight ahead of the season-opening race in Australia in mid-March due to a knee problem.
"I did a hell of a lot of training over the winter in terms of diet, really limiting my intake, but in a safe way," said Button.
"But that's all gone out of the window in January. I haven't trained for three weeks because I've had an infection in my knee.
"And when you are sat on your backside all day like on Tuesday, you want to eat, so it's not been the easiest.
"Right now I'm not as light as I will be at the first race, but I'm definitely going to lose another kilo by race one, and it's going to be tight."
At least Button was happy with the baseline of his new car as opposed to a year ago, when McLaren built one that failed to claim a podium for the first time since 1980.
On this occasion, a relieved Button said: "I enjoy driving this car.
"Our first day last year we were quick, but the car had its flaws and we could see that. This time we don't have those issues.
"The basic car itself is good and it works. No scary things with it. I'm really looking forward to getting back in it."
In sharp contrast, reigning champions Red Bull and Vettel, along with engine partners Renault, suffered a second successive day of woe.
Vettel departed the circuit early on Wednesday after completing a total of just 11 laps over the two days due to issues with Renault's new systems.
A frustrated Vettel said: "With such big rule changes it's usual to have teething problems, and that's what tests are for, to sort those issues out."