Covering over 100 laps for the first time in his fledgling F1 career, Max Verstappen says he is feeling "more comfortable" in the STR.
Putting in qualifying style runs on the super-soft tyres in the morning's session at Barcelona, Verstappen posted a best time of 1:24.739 on Saturday.
That time saw him top the timesheets at lunch although he was later edged by Pastor Maldonado in the afternoon.
Verstappen, though, was still very happy with his day's work, which saw him amass 129 laps of the Spanish circuit.
He said: "A good session and the first time I’ve done over a hundred laps in a single day. We got through a lot of items on the job sheet, with some faster runs in the morning.
"The car felt really good and it seems we are improving the car step by step.
"Compared to after the first test in Jerez, I feel more confident and more comfortable with the car, as I start to learn about every aspect of the job.
"Now, I’m really looking forward to seeing what the engineers can come up with for the final test here next week."