The Spaniard ended his first experience of an F1 car with the second fastest time of the day at Silverstone - just a few hundredths of a second adrift of Toro Rosso regular Daniel Ricciardo's best effort.
But despite the impressive speed, Sainz thought there was little point reading too much into a lap time that has propelled him in to the F1 spotlight.
"This test is just a test - and it is not because of doing this test or doing a good job in this test that I am one day going to be racing in Formula 1," he said, when asked by AUTOSPORT whether the day had changed his prospects.
"If I am here, it is because Red Bull has chosen for me to try to do a test. But this test does not mean anything.
"The only thing I hope is that they are happy with my performance and I am mature enough to do more testing, because of course after doing one test you need to do more. You always want more and more."
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Sainz also reckoned it premature to even claim he had done a good job, and said the only thing that really mattered to him was what his bosses thought.
"I think it is more for you, and the Toro Rosso guys, to judge my position," he said. "It is not up to me.
"You always prefer to see yourself P2 than P9, but for me maybe it was two seconds off the pace. I am not sure. It is difficult to judge, as there are so many factors that can affect it: engine mixtures, or downforce.
"This car can be eight seconds faster or eight seconds slower – there is such a big range of fuel loads. I leave it up to you guys, because I don't want to think about how good I've done until they tell me."
'BEST DAY OF MY LIFE'
Although eager to play down the significance of his time, Sainz could not hide his sheer delight at having experienced an F1 car.
"It is completely unbelievable, the feeling this day," he said. "If there is a track where you are going to get the feeling of an F1 car, or the potential of an F1 car, it is here at Silverstone.
"For me, this day has been the best day of my life. It is a dream come true. It is very important for me and, after jumping out of the car I wanted to thank every single mechanic and every single engineer – they have been part of the most important and happy day of my life."
- Toro Rosso
- Daniel Ricciardo