Alonso, who won in China last weekend, set the fourth fastest time in second practice.
"It was a positive day and we are reasonably pleased with how the car behaved," he said.
"We had no major problems and more importantly, we didn’t have to make any radical changes to the F138 to adapt it to this track and its long straights and slow corners".
The Spaniard is unsure as to whether he will keep using the new upgrades brought to Bahrain.
"The initial feelings are good and I think we can be up in the leading group tomorrow, a group that is all within the same two, maximum three tenths. We have not yet decided if we will use the aerodynamic updates we tried in the morning. It is not yet completely clear how they worked and in the afternoon, we concentrated on comparing the two tyre compounds we have for this race."
A satisfied Felipe Massa was sixth fastest.
"Everything went well today, even if it’s not easy to say at the moment where we will be on Sunday," said the Brazilian.
"Car balance was good and it behaved well in all conditions, even if we suffered a bit with tyre wear, especially at the rear. It’s pretty normal to have significant degradation in such high temperatures and on this front, we have a lot of work to do to prepare for the race."
"On the Medium tyres, the F138 worked really well, even over a long run, but the key on Sunday will be managing the degradation of both compounds, as well as deciding on whether or not to use the updates we tried here this morning."