Sainz’s Ferrari briefly lost power after the red flag stoppage in Q1, but the issue resolved itself allowing the Spaniard to safely make his way through the first two qualifying stages.
He went onto qualify sixth, two places ahead of teammate Charles Leclerc.
Sainz described qualifying in Mexico as “too stressful” and “very complicated”.
“Yeah it’s just been too stressful for my liking,” Sainz said. “It’s been a very, very complicated session. I lost the feeling with the car in Q1, I also lost the feeling with the medium tyres, the car was a bit tricky so actually I never really did a clean lap until Q3, run two.
“Even the lap, because I was so much out of rhythm in the previous laps, it didn’t feel particularly special compared to FP3 or FP2 laps when I had a much better feeling.
“When I finished the lap, I thought I was going to be P10/P9 and suddenly they told me I was P6 and I was a bit happier. We expected this session to be a bit easier than it was.”
Sainz admitted that he expected to beat Pierre Gasly to fifth on the grid despite the AlphaTauri driver benefitting from a tow from his teammate Yuki Tsunoda.
“Well they’ve been playing around with tows which is a couple of tenths around here and they’ve been on our pace the whole weekend,” Sainz added. “If you add the two-tenths of the tow then normal they are two or three tenths ahead.
“Apart from that, honestly, I expected to be ahead because in FP3 I was flying, I was really comfortable with the car and in qualifying, I wasn’t. Something to analyse there and something to keep looking for.”
Leclerc could only manage eighth on the grid and he was left disappointed by his own performance across the weekend.
“I am disappointed in myself today,” he said. “It’s been a very difficult weekend for me. FP1, FP2 and FP3 I have been struggling but then in qualifying, I could see the light at the end of the tunnel with the car getting to where I wanted it to be and we were very competitive.
“When I needed to do the lap in Q3 I didn’t do it because of mistakes from my side. It’s like this.”