Leclerc's team-mate Carlos Sainz had led the final morning session from Red Bull's Sergio Perez before running was interrupted by a broken drain cover for the second day in a row.
Because repairs halted the test for 1h17m, the decision was taken to cancel the planned one-hour lunch break and carry on all the way to the chequered flag at 7pm local time.
There was early trouble for McLaren, which added to its testing gremlins this week with an apparent clutch issue for Lando Norris. Sauber's Valtteri Bottas also ran a restricted programme while his team worked on the floor and rear end of his car.
In the afternoon Max Verstappen took over from Perez in the RB20, while Leclerc replaced Sainz in the scarlet Ferrari.
Leclerc soon went third-fastest behind Sainz and Perez until Verstappen put down a 1m31.058s, followed by a 1m30.763s on used C3 tyres 25 minutes later.
Leclerc replied with a 1m30.409s to go faster, but used the C4 compound that is a significant step softer and won't be used at next week's Bahrain Grand Prix at the same Sakhir circuit.
With two hours left to run the Monegasque driver took on another set of the red side-walled tyre to clock a 1m30.322s, going four tenths clear of the world champion.
Heading towards the final hour most heavy hitters switched their focus to longer runs on the hardest tyres.
Source: Autosport