Leclerc earned second place in Jeddah while Sainz was over half a second behind in fifth. However, the Spaniard will start fourth as Leclerc drops to 12th thanks to his power unit grid penalty.
Sainz said he struggled with the SF-23 in the first sector of the lap.
"In FP3 I was feeling OK and I had a decent feeling with the car," he said when asked by Autosport about the qualifying deficit. “But for some reason sector one was a main, big weakness for me today.
"I couldn't get the tyres to give me the feeling to push in Turn 1/2 and the high-speed, and every time I was pushing a bit much I was snapping and having big moments.
"So in the end, I was three or four tenths off - which is a lot for one sector - so there's clearly something there that I didn't understand, or didn't put together.
"I was playing a lot with prep laps, out-laps, and never switched on the tyres for sector one. Sector two and three I was actually OK, so without this problem tomorrow I think we could have a good day."
Regarding the car balance he added: "It's not the most predictable rear end I've driven in my life. I'm not sure still if it's the car or the tyre preparation in sector one. We still have to look a bit into it."
The Ferrari drivers were a low-key ninth and 10th on Friday, and Sainz confirmed the team had been concentrating on long run performance during the weekend, rather than going for times.
"Yeah, we are putting a lot of focus on preparing for the race, so maybe we could have arrived a bit more prepared for quali, and maybe this cost me a bit," he said.
"But I think Charles showed what the car is capable of so, if we had had a clean day, we would be starting P2 tomorrow.
"We didn't have a great quali, I had to use two sets in Q2 because of the moment, and then I didn't have two new sets for Q3. We actually decided to save one."
Sainz agreed that Leclerc’s pace was a boost for the team after a difficult start to the season.
"Yeah, it's very good news. It's still not where we want to be because we would like to be P1 and P2, it still shows that we have a few weaknesses here and there. But also Max [Verstappen] wasn't in the fight. I still think we can have a strong race day tomorrow, and go from there.
"I think we have the potential, I think the race pace should be better. So let's stay patient, but I think we can target a podium tomorrow."
