Alonso elected to miss the first pre-season test in Jerez but returned to set the third fastest time of the day.
"I feel reasonably pleased with this first day and I can say I share Felipe’s opinion that the car is on another planet compared to the one we began with on the first day of testing last year," he said.
"The programme for this first session in Barcelona is aimed mainly at improving reliability, with the target of completing more than a hundred laps every day."
Alonso completed 110 laps today, more than any of his rivals.
"Then, at next week’s test we can concentrate on performance. Tomorrow, we will complete what we were unable to do today and we will focus on car set-up, to adapt it as well as possible to the tyres. I knew from looking at the Jerez data what sort of car I would find here today, as in fact we know more or less what we will see at the first race in Australia, in terms of what new parts we will have and how many tenths they will produce. This is down to all the work done over the winter in the wind tunnel and the numerous tests which confirm that the parts that are on their way will give the expected results."