Sainz was stuck in the garage until the final minutes of the session after reporting brake-by-wire problems on his Ferrari.
He came back out in time to attempt one flying lap, which propelled him from last to first.
"It was a very stressful Q1 in these conditions," Sainz smiled. "Obviously, not ideal to be able to do only one lap in Q1 and to put it on P1 was a pretty heroic lap.
"Without knowing how the track was at the time, having no idea how much grip to find, to put it there with only one lap was a pretty good feeling."
Sainz ran out of fresh soft tyres for Q3 and had to settle for fifth on Saturday afternoon's sprint grid.
"I was quite quick in Q2 on the used tyres and also in Q3, but just due to the problem we had yesterday in Q2 we didn't have any softs left, which we know around here is three to four tenths, so that's why we are P5," Sainz added.
After heavy overnight and morning showers, the Red Bull Ring had dried up just in time for the F1's sprint shootout, but reduced grip levels and damp kerbs still made qualifying for the second sprint of the 2023 season hazardous, with teams having headaches on which tyres strategy to use with the sets of news slots they still had available.
Source: Autosport