Grosjean completed 54 laps of the Jerez circuit, finishing in third place.
"The feeling is good from the E21; it feels very similar to the E20 from behind the wheel, even if it does look different from the outside," he said.
"Once again Enstone has produced a car that is quick from the very first lap. We spent a little bit more time than ideal in the garage today, but that’s normal in testing. I’m happy we completed 54 laps, and even happier that all of them were quick. We now have a good idea of the new tyres and we’ll have some challenges from them; the grip level is there but performance does fall away with graining. That said, Jerez is a very specific track in this regard. The speed came easy today and we didn’t respond to the other cars going faster in the afternoon. We should go even better tomorrow."