For the first time ever, GP3 racer George Russell was behind the wheel of Mercedes' W08, ending the day P4 about half a second behind team leader Valtteri Bottas.
Russell put in 119 laps as he focused on aero testing and some brake system work.
"Today was incredible, "enthuised the 19-year-old.
"I have been looking forward to this day for so long. It's every young driver's dream to drive a Formula 1 car.
As soon as I got on track, I straightaway understood how much grip and downforce this car has got - it is amazing. 48 hours ago I was driving a GP3 car around the same track, but today was totally different.
"Luckily for me I had done lots of simulator work with Mercedes, so I knew what to expect. The biggest difference is the power steering in the Formula 1 car, and I could feel that as soon as I got out on track."
Mercedes W08 was obviously a different proposition for Russell compared to his GP3 racer.
"The braking potential is enormous. I was expecting the speed on the straight and the downforce in the corners, as everyone does. But I don't think anyone quite understands the braking power of this machine.
"In GP3 we enter into Turn 1 at about 260 or 270 km/h, braking at the 100 metre board. Today, I was going in at almost 300 km/h, braking 25 metres after what I would do in GP3.
"Today mostly consisted of longer runs with higher fuel loads. We wanted to see the degradation of the tyres, and mainly trying to work for the race simulations. I was quite happy with my performance.
"For my first time in the car, completing 119 laps and not being far from Valtteri - I'm pretty pleased with that. "