"Holy s----," Sainz said on the radio. "I didn't expect that one!"
It was pole position at the 151st attempt of his Formula One career, and that outcome had looked unlikely going into qualifying. Sainz had been quickest in Friday's second practice session, but his car looked completely different on Saturday, as he struggled with bouncing at high speed.
A heavy downpour created an unpredictable qualifying session, with times continually improving until the end.
"Honestly I had no idea where I was going to qualify because the conditions were so changeable that I didn't even know what was the lap time to beat in Q3," Sainz said later. "I didn't know what lap time I had to do on that lap to go pole.
"I was convinced that it was not going to be enough. But then it turned out to be enough, and I was pretty happy to hear that."
The Silverstone pole could be a pivotal moment in Sainz's season. His form to this point can be described as erratic at best, with some people in the paddock doubting Ferrari's decision to extend his contract to 2024 while he was underperforming in a title-contending race car.
His struggles are hard to understand in the context of 2021, when he outmatched teammate Charles Leclerc across the year. Things have gone the other way this season. Leclerc has claimed two race victories and six pole positions -- Saturday's pole position was Sainz's first of either.
The next achievement to tick off the list is an F1 victory, and there have been encouraging steps in that direction recently. Sainz finished the Monaco Grand Prix in second position, convinced a slow lap behind Nicholas Latifi cost him the victory, and then finished the Canadian Grand Prix just 0.9s behind Max Verstappen, having chased him home for the final portion of the race. He missed out on a maiden F1 win in similar circumstances behind Pierre Gasly at the 2020 Italian Grand Prix.
To win on Sunday, he will have to contend with Verstappen alongside him on the front row and Leclerc starting directly behind him. If it stays dry, as the forecast suggests, Sainz is confident he has the car to convert pole position into a first career victory.
"I think we've been quick in every race this year, so there's no reason to believe that we shouldn't be quick enough," he said on Saturday. "It's true that the Red Bull is always as quick as us, sometimes quicker, sometimes a tenth slower.
"But in the end, those races where it is so tight, it's won by the details. What we need to focus on is getting the details right with the strategy, the tyres, the management. If we get these things right, if we have track position, we should try and stay ahead.
"I think, working well together, we can beat the two Red Bulls."