The Spaniard was outperformed all weekend by Ferrari team-mate Charles Leclerc, who took pole for both the sprint and the Grand Prix, and finished second in the shorter race and third in the main event.
Sainz in contrast had to settle for fifth place in both after experiencing a difficult weekend that was exacerbated by the lack of practice running and the imposition of parc ferme after FP1 preventing set-up changes.
In the Grand Prix he lost out to countryman Fernando Alonso, who muscled past after the resumption following the safety car period.
"Seeing where Fernando finished, I think he was always going to finish ahead of me today,” said Sainz when asked about his race by Autosport.
“But yeah, nothing new. Really, the first stint on the medium, I looked like I found a bit of pace back.
"And then when as soon as I put the hard on my struggles of the weekend appeared again, and I just had to bring home a P5, always very on the limit of crashing, or feeling like I was losing the car.
“So [it was] a pretty long race and mentally stressful after the struggle of the weekend. But I brought home the P5. I'm sure we will find out why this happened this weekend. And I'm sure in Miami, we'll be back on form."
Source: Autosport