The Spaniard ran a comfortable third in the early stages of the Miami event, but fell away from leaders Max Verstappen and Charles Leclerc after the midway point.
While it appeared from the outside he may have struggled with the balance on the hard tyre, Sainz has revealed that the main factor for him was actually his physical condition.
Having taken a knock in his hefty Friday practice crash at Miami, he was also suffering from a lack of race fitness – having retired early in both Melbourne and Imola.
In the Australian GP, he was out on the second lap after spinning off, while in Imola he did not even complete a racing lap after being tapped from behind by Daniel Ricciardo at the first chicane and getting stuck in the gravel.
The back-to-back retirements left Sainz short on mileage and meant he had not completed a full race distance since the Saudi Arabian GP.
“There were no issues with the balance,” he said when asked to explain his form in the Miami race.
“The balance was actually obviously tricky because we couldn't keep up with Max and the Red Bull.
“But, on my side, it was more me feeling not 100 percent basically towards the end of the medium stint.
“In the last five laps, I started to get a bit tired on the neck and I couldn't push 100 percent coming from the crash of Friday.
“I will not use it as an excuse, but it was just also maybe lacking in the last two races to get the neck and the body used to these F1 cars.
"Obviously I have been a month without a full race distance and probably that, combined with the crash on Friday, just made me not be able to push 100 percent up until the safety car more or less.”
But while feeling he could not give everything in the race, Sainz admitted it was important for him to finally make it to the chequered flag after his recent woes.
“It's not so much a relief – but it’s needed,” he said. “I think I needed to complete a race distance, to get the body back to shape, and also get the feel for the car on used tyres, high fuel.
“I was still doing a couple of mistakes out there during the race, just because I was trying the car and trying myself out there.
“The important thing is that we got a full race in. At some stages of the race I was pretty quick and also the battles, and the feeling with the car in battle with Checo [Sergio Perez], you know what to do with the battery and with the tyres.
“I think it gave me a good understanding of what to do in the future.”
