Bearman was called in after Sainz was ruled out of the rest of the Jeddah event due to appendicitis, with the Spaniard requiring surgery.
The 18-year-old had to abandon his Prema F2 seat, having secured pole position for Saturday's feature race, and only had FP3 in which to find his feet before heading into qualifying.
He made a strong impression with a confident performance that saw him just miss out on Q3 in 11th place.
Revealing the advice given to him by Ferrari team principal Fred Vasseur, Bearman said: “Fred, he doesn't say much, but what he says, he means. He was very clear to me that I have to build it up, step-by-step. Don't try to be a hero.
“Especially after FP3, which was already a good session, his goal for me was just to keep going like that, and build it up step-by-step.”
Although Sainz had felt unwell on Wednesday and Thursday Bearman insisted that he had no idea that he might be required.
“Honestly, no,” he said when asked by Autosport if he’d been put on standby. “I woke up this morning fully prepared and fully ready for my [reverse grid sprint] F2 race, starting from P10.
Source: Autosport