The McLaren driver had looked strong throughout qualifying, but nearly threw away all his efforts on his final run in Q3 when he almost lost control of his car at the first Lesmo corner.
Knowing that he needed to make up for the time lost, Sainz risked everything over the remainder of the lap before pulling off a slot on the second row – and the effort left him drained afterwards.
“On the last lap I nearly messed up,” explained the Spaniard. “I had a big moment in Lesmo 1, and I nearly lost it. After that, I had to drive like this [on the edge] and I'm actually shaking a bit because in Parabolica and Ascari I had to really concentrate.
“I felt like I had nailed Q1, Q2 and Q3 run one. So I was waiting for Q3 run two, and the lap didn't start very well.
“I was already needing some very strong final few corners and I really went for it. I risked a lot and, after the dive I did into Ascari, I was a bit like 'woah, that was way too late!’"
Sainz’s performance was a surprise with many having expected Renault and Red Bull to be battling it out as best-of-the-rest behind Mercedes.
But the team’s decision to opt for an ultra low drag solution this weekend appears to have paid dividends, although Sainz is still unsure of the exact reasons why McLaren is so strong at Monza.
“I would like to be able to answer that specifically, but at the moment I can't,” he said. “We've just been very quick today.
“Honestly, I felt since Q1 we had the upper hand on the rest of the midfield and I just needed to put some clean laps together. But yeah, we're running very low downforce which on our car last year didn't work. Our downforce was just falling off whenever we ran very little rear wing.
“This year somehow our car is a bit more robust at running lower downforce levels and in Spa it started to pay off. Here in Monza, we have a strong car under braking, which means we can brake late, and we have a strong car for Parabolica and Ascari, which obviously helps.”