The Spanish driver had qualified just a tenth slower than Leclerc but dropped back at the start of the race whilst the Monegasque jumped Valtteri Bottas' Mercedes.
With track position critical in Spain at a circuit where overtaking opportunities come at a premium, Sainz eventually finished 20 seconds adrift of Leclerc and behind McLaren's Daniel Ricciardo, costing the team constructor's points.
“[It was] a bit frustrating because in the laps to the grid I felt the car was behaving really well and I was very comfortable," said Sainz.
“I knew a good start was needed and unfortunately I couldn’t make it happen. Not exactly sure what happened on lap one and what I could have done differently to not lose a couple of positions.
“From there on it was a race of staying behind people, trying to push to overtake them but not having enough top speed to quite make it stick. I wish I could have done the race in a bit more clean air and it could have been much better.”
Sainz did manage to make a move on one McLaren although ex-team-mate Lando Norris was shown the black-and-white flag for bad sportsmanship after jinking in defence from the Ferrari.
The Spaniard commented: "He moved a bit on the limit, late, I just reacted enough to make it stick.
“Daniel we were on the same aged tyres and I was 3-4 tenths quicker but that is not enough to overtake around here. With the power unit they have in the back of their car this year, the acceleration they were getting was quite impressive.”
On whether he now feels comfortable with his Ferrari after his winter switch from McLaren, Sainz added: “I feel quite up to speed.
“[There are] still a couple of things to keep looking at and improving. I feel like I could have qualified fourth with a good lap, I felt I was pushing Charles quite nicely through Q1 and Q2.
“In clean air, I was very quick, so the pace is there it’s just putting the whole weekend together, so putting that Q3 lap together, the start, lap one, which is what I used to do very well at McLaren.”
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