After Ferrari reserve Robert Shwartzman ran in the car during FP1 and classified 19th, Sainz clocked 16th in FP2. Team-mate Charles Leclerc finished 16th and then 11th.
Both Scuderia drivers have called on the team to make major gains overnight, with Sainz supposing that Ferrari has again been caught out by a high-downforce track.
He suggested Ferrari's larger rear wing specification could not match the loads generated by rivals. This reminded him of the Hungarian GP when he qualified 11th.
He told F1TV: "Unfortunately, we don't look very competitive yet. We're going to need to find some lap time, some downforce, some balance because today was a tough day for the team.
"I think we saw something similar in Hungary where we go to higher downforce tracks, for some reason we don't pick up as much downforce maybe as the others when we go to the bigger rear wings.
"A bit of homework to do on our side [to understand] why we slide more than the others.
"If we want to be fighting for a podium [in qualifying] and on Sunday, it needs a very big jump in performance."
Source: Autosport