The American outfit are preparing for their second season in F1 and will be hoping for much better this time around as they tailed off after a fast start to 2016.
New regulations mean new regimes for the drivers and the changes to the rules are set to make things more demanding in 2017 and Grosjean admits he has had to work harder in the off-season.
The 30-year-old says he felt more comfortable than he thought he would at the Circuit de Barcelona-Catalunya and believes his hard work has paid off.
"I didn't feel too bad. But let's put it this way, if I'd been in the same condition as I was last year, I would have been destroyed," said Grosjean. "We pushed really hard in our training. We may actually have overdone it.
"It's not as bad as we thought it would be. The cars are going to be challenging and some of the races this year are going to be epic, especially where it’s warm with a high-speed track, it'll be very hard on the body.
"I like the challenge and I like to think that we can always get more prepared and better trained. It felt good as we did the proper training. The cars are much harder to drive than last year."