With less than three minutes on the clock, Leclerc degraded Pérez to second, but the Mexican responded in the final seconds to reach the top with a lap of 1m12.476s, 0.041s faster than Leclerc.
Williams’ Nicholas Latifi and Alpine’s Fernando Alonso set their first times in the third and final F1 training session on the principality’s sunny streets.
They were soon joined by McLaren driver Daniel Ricciardo, who tried to make the most of FP3 after missing the entire FP2 on Friday to fall into the pool section on his first lap.
But all eyes were on the pre-event favorite, Leclerc, who has joined the track after eight minutes in soft and took charge of his teammate Carlos Sainz with an effort of 1m14 .008s.
The first laps of Red Bull were four tenths of the pace of their title rival, Max Verstappen briefly took third place ahead of Perez, with Alonso at the start of fifth place.
Leclerc’s second effort was even faster, lowering the reference to 1m13.647s. A cycle of exchanges between Ferrari and Red Bull began over Pirelli’s softer compound as the track’s evolution continued to slow down, with Pérez and Leclerc even setting identical times.
At 30 minutes, Leclerc had climbed again with a 1m12.885s, two tenths of his best time in FP2.
Meanwhile, McLaren’s Lando Norris briefly regained his best resting place in fifth place, which he had held all Friday, only to lose it to AlphaTauri’s Pierre Gasly before halfway.
Mercedes struggled to pick up the pace from the start, with George Russell and Lewis Hamilton falling in seventh and 10th place, respectively, while Hamilton seemed baffled by the 1.2-second difference he faced.
Sainz went straight to Ste Devote as he tried to match Sainz on the Monegasque home court, as did Aston Martin’s Lance Stroll.
During a half-session calm in action with just over 20 minutes to go, Norris found a clear clue to go third, which was soon overtaken again by Gasly, which suggested that the best teams still they had time to go back to find the last third.
They did so properly, with Leclerc and Perez exchanging the top spot until Perez finally secured the lead of the session ahead of the checkered flag.
Sainz took third three-tenths behind Perez and half a tenth above Verstappen, which he had seen as one of his traffic-ruined thrust laps.
Gasly continued to hold Norris in fifth place, and Hamilton finally found some pace to finish seventh, though still nine-tenths of the way ahead of the leaders.
Kevin Magnussen was a strong eighth in the Haas, who has yet to receive a significant improvement.
Russell and Alonso completed the top 10, followed by Yuki Tsunoda, the second Haas of Mick Schumacher and Vettel.
Valtteri Bottas has continued a difficult weekend for Alfa Romeo in 14th place, just ahead of Alex Albon in the first of Williams’ cars.
Ricciardo was only able to finish 16th after losing time on the track, with Esteban Ocon of Alpine falling to 17th after a difficult session.
Stroll was 18th for Aston Martin, while Zhou Guanyu and Latifi propped up the timesheets in 19th and 20th, respectively.
Traffic remained a problem, with Vettel furious at an upcoming call with Sainz.
There have also been incidents between Verstappen and Zhou, Perez and Russell and between Sainz and Stroll, for which all pilots have been summoned by commissioners.
Vettel’s teammate Stroll contacted the wall at the exit of the pool section after grabbing too much sidewalk, damaging his front spoiler in a session that would otherwise not be clean. .