Fresh from inking his contract extension at Red Bull and his victory in Monaco last time out, the most successful-ever Mexican driver set the pace with a 1m45.476s, a little over 4s slower than Leclerc’s pole time from the 2021 edition of the race .
Leclerc ran a tenth adrift, while defending champion and points leader Max Verstappen ran to third ahead of Carlos Sainz and the bouncing Mercedes of Lewis Hamilton.
All the drivers ventured out on the 3.7-mile city street circuit within the first two minutes of the hour, the majority electing to run on the C4 medium-compound Pirelli tyre.
The Red Bulls were the first to complete laps in under 1m50s, with Verstappen taking six minutes to move top with a 1m48.574s effort to sit nine tenths pretty over his teammate.
But after only seven minutes, yellow flags morphed into a virtual safety car to interrupt the running in response to a bizarre car failure for Mick Schumacher.
The German was forced to stop at Turn 12 on what was only his third lap.
The Haas VF-22, rebuilt after its £1million crash in Monaco, was pictured gushing pink liquid from the right edge of the floor.
This is suspected to be radiator fluid, after his engineer reported a “torque protection” trigger following a water pressure spike. He was benched for the rest of the session.
Verstappen was able to improve after 12 minutes, bringing the benchmark down to 1m48.390s with what were at the time, the fastest first and second sector of the day.
But Fernando Alonso was the pacesetter through the flat-out final sector of the lap, having been pulled along by a significant tow.
The VSC was called into play yet again after only a quarter of an hour, this time for five minutes to attend to Nicholas Latifi stopping with a power failure at Turn 5.
Like Schumacher, the Canadian was forced to sit out the remainder of the hour.
Yet again it was Verstappen who ran fastest when the track conditions cleared, the Dutch racer improving to a 1m46.932s to pip Leclerc by night on seven hundredths of a second.
Verstappen lapped even quicker aboard his RB18 next time around, dropping to a 1m46.477s to sit half a second ahead of chief championship rival Leclerc.
Perez would climb to second on the medium tire shortly after as both Ferraris stuck to the slowest-available C3 hard compound early on.
With 25 minutes to go, Verstappen once again set the pace as he became the first person to dip into the 1m45s, finding seven tenths over Perez.
But then Perez came to the fore with a third of FP1 to go. He gained massively in the first sector and cut it fine with the Turn 15 outside wall to nail a 1m45.476s.
That ultimate time gave him a one-tenth cushion over Leclerc at the end of the session, as Ferrari returned to hard tires and neither Red Bull improved when shod with softs during the final 10 minutes.
Verstappen’s 1m45.810s lap put him three tenths adrift as Sainz, one of many to struggle with porpoising on the long straights, clocked the fourth-quickest time.
But Verstappen’s session was somewhat hindered by a DRS flap that kept flapping wildly at higher speed and then, having adjusted settings in regard to engine braking, he span at Turn 15 when the rear pushed wide.
Alonso ran fifth fastest ahead of Hamilton, who complained of a distracting beep in his headset after enduring violent porpoising aboard the Mercedes W13.
Yuki Tsunoda split the Silver Arrows in seventh, with George Russell in eighth while Pierre Gasly and Esteban Ocon completed the top 10.
Lando Norris ran to 11th for McLaren, beating Lance Stroll who twice whacked the Tecpro barrier on the exit of Turn 15 – the second occasion forcing him to pit for the team to inspect any damage.
Kevin Magnussen clocked 13th over Sebastian Vettel as Valtteri Bottas led the Alfa Romeo charge.
Behind Alexander Albon, Daniel Ricciardo was only 18th.
