Saturday afternoon's qualifying was held in soaked conditions, but as the top 10 made its way into the final Q3 shootout the track's racing line started drying up by the second.
That meant that drivers had to continue circulating to get a fast lap in as close to the end of the session as possible to find the best conditions, and it meant virtual pole frequently changed hands between Ferrari's Charles Leclerc and Red Bull's Max Verstappen.
But with his final lap Sainz grabbed his maiden F1 pole by just 0.072s over Verstappen, with Leclerc three tenths behind in third and the second Red Bull of Sergio Perez a distant fourth.
Sainz admitted he was shocked that his lap was good enough for pole, because he thought it was "nothing special".
"It was a good lap, but I was struggling a lot with the standing water on the intermediate, there was a lot more standing water, even on the racing line, and it was very easy to get snaps and lose the lap" Sainz said.
"It was also more difficult to get temperature into this intermediate for this condition.
"In the end I put together a lap that I didn't think was that special just to be on the board and it was pole position which came as a bit of a surprise."
With his first pole in the record books, Sainz is now chasing a maiden victory in F1 and believes he has race pace to pull off a double breakthrough on Sunday.
"I think so. The pace has been there all weekend, except for FP3 where we had some issues that we that we think we have corrected for qualifying, but if I base myself on my FP2 pace then we should be in a good position to try and hold on to it.
"I'm sure Max and Charles will put a lot of pressure on, but I will try my best of course."
Leclerc spun on his final lap and therefore "didn't deserve to be on pole", but said he was happy to see his teammate take his first pole.
When asked if he was disappointed, Leclerc said: "Yeah I am, but happy for Carlos. He did a great job today.
"I spun in that last lap, I knew it was the lap where I had to put everything together and I didn't as a driver, so I didn't deserve to be on pole.
"But P3 is still a good position to start tomorrow's race. And hopefully, putting everything together, we can come back."
"It will be interesting to see whoever is going for one stop or two stops, but hopefully we will make the right choice and we can come back to where we want to be."
