Pierre Gasly was left feeling frustrated after he crashed the RB15 in Barcelona on Tuesday afternoon.
The Frenchman brought out a red flag after he ended up in the barriers at Turn 10 during the session. Gasly missed the last hour and a quarter of running due to the incident, bringing his day to a premature end.
"Out of Turn 10, I went on the power upshifted to fifth and lost it. It was quite annoying to lose the last hour of testing today," Gasly said.
However, he retained some optimism from the session: "Overall it was really positive, I was really happy to be back behind the wheel after the winter break.
"The first feeling is good, I did a lot of things with the team, a lot of positive things. Of course, we have a lot of work to do during the next six days, but the first one was great.
Gasly's fastest lap placed him seventh in the final results. It was the newly promoted Red Bull driver's first day behind the wheel following the winter break, and he managed to clock 92 laps in his Honda-powered challenger.
"So far it has been positive, we've tested a lot of different things, trying to understand the car a bit more and the package we have," he said. "But for my first day in the car, I felt very comfortable, so pretty positive.
"I think it's too early to talk about expectations at the moment, we need to understand the overall performance of our package. It was important for us to have some feedback for this car, bit so far everything is good. We tested some positive things, some negative too, but we need to understand a bit more before having any expectations."