Leclerc and Max Verstappen traded places atop the timing sheet in FP1 and FP2, although the Red Bull will drop from contention somewhat come Sunday as he serves a five-place grid penalty for taking a new Honda engine in Sochi.
Mercedes had the third-quickest car in both sessions, suggesting that a fourth consecutive win could well be on the cards for the Scuderia, but both of the team's drivers left the track on Friday looking for more.
"There is still quite a lot of lap time to be gained on our side," Leclerc said.
"The balance was not exactly how I wanted it to be, especially in the qualifying runs, but the race pace looked very strong, so that's good. It's mostly the balance throughout the qualifying lap."
Vettel echoed his team-mate's sentiments, although his assertion that the Red Bull looked the strongest car on-track will be music to Ferrari fans' ears as they bask in the glory of three successive victories for the first time since 2008.
"It was a bit strange. We tried some stuff, and we need to decide what to keep and what to get rid of," Vettel said after Friday practice.
"I think we can improve, especially in terms of balance, we'll put it altogether and it should be a better day tomorrow.
"How much time we can manage? It's difficult to know. But tomorrow we are on the good side of things."
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