Yesterday Rossi finished third fastest overall behind Honda riders Dani Pedrosa and Marc Marquez ahead of Sundays Dutch Grand Prix.
The Italian explained that though the frame was tested initially at Aragon he had not spent enough time on the bike to get used to the changes and know if they would be good or not.
But after Thursday’s two free practices Rossi was very satisfied with how the Yamaha performed.
"I'm very happy and satisfied about this first day," Rossi told MCN.
"It was an important day, especially for the weather conditions. Very great conditions and you have to use this dry time to the maximum to understand the setting of the bike because you never know about the weather at Assen.
"At the same time we had the chance to understand about the new chassis from the test in Aragon. I'm happy because already from this morning when I put the new chassis I improved my lap time, my pace and I felt good with the bike."
Though things felt good for Rossi, he was wary of the two Honda's of Pedrosa and Marquez who were both faster than him in FP2.
"At the end I put the medium tyre and I did some very good lap times, also with the pace I'm not so bad. So in general it is a positive day. The track is great, the bike worked well.
"The only problem are the Hondas, because looks like both Hondas are very strong and we are always very worried about the Hondas because they have the old chassis and both the riders Marquez but also Pedrosa are very fast, very strong.
"So it's good to stay with them, be in third position and be a small difference from first," he added.