Charles Leclerc led a Ferrari 1-2 in FP2 on Thursday, but the local hero is taking a cautious approach to Saturday's qualifying, suggesting that both Red Bull and Mercedes have more margin to improve than the Scuderia.
Leclerc and Sainz closed out the afternoon practice session on top, a performance that surprised Ferrari's rivals and that came after a morning session in which the Monegasque was sidelined after just four laps due to a gearbox failure.
Leclerc best flyer on the soft tyre was 0.112s faster than Sainz's best effort and 0.390s ahead of Lewis Hamilton.
"I was quite surprised that it ended that way because Monaco, here especially, it’s very important to do as many laps as you can," said Leclerc. "Unfortunately, with the issue I had in FP1 I had only done four laps."
But the 23-year-old got down to business and on top of his machine at the outset in FP2. However, he isn't sure he can extract enough speed from Ferrari's SF21 to maintain an edge over Mercedes and Red Bull's contenders.
"I felt quite at ease with the car in FP2," he added. "I managed to put that lap, there’s still a bit of margin. But on the other hand, I’m sure that Mercedes and Red Bull have more margin than we have.
"So we shouldn’t get carried away too much. We’ll have one day tomorrow to work hard and then let’s see where we end up.”
"I want to wait for Saturday for now. It’s looking good but for now it’s looking a bit too good to believe it.
"So let’s wait and see. On Saturday we’ll know exactly where we are in qualifying and then we’ll go from there for the race."
Gallery: The beautiful wives and girlfriends of F1 drivers
Keep up to date with all the F1 news via Facebook and Twitter