“A bit surprised with how much we struggled with the hard tyres after a very strong stint on the mediums,” he said.
“You do one good fast lap and then the next lap you are three-tenths slower with the tyres that were just very, very tough.
Ferrari had successfully executed the undercut on Alonso to place the Madrid-born racer in a net third position.
But Sainz’s ensuing struggles on the hard compound meant that he soon fell behind the Aston Martin once more before he was then passed by Russell’s Mercedes.
“We tried the undercut on Fernando, obviously it worked, but it meant that by pushing the tyres for three or four laps for the undercut it meant for some reason right now with our car we have zero flexibility with how much we can push and not push.
“I had to stay under the limit for the rest of the race if I wanted to make it to the end. With the wind conditions, how tough we are on tyres that hard stint just became too long for us.
“I am surprised as I thought we would be better but this race showed us that we still have some work to do.”
Sainz’s Ferrari team-mate, Charles Leclerc, suffered even worse. The Monegasque driver’s crash late in Q3 placed him P7 on the grid and resulted in him getting caught up in a tight midfield battle throughout Sunday’s race, eventually wounding up in the same spot he started.
Ferrari’s haul of 16 points from the Miami GP saw the Italian outfit remain fourth in the Constructors’ Championship.