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The pair were split by just 0.029s at the top of the timesheets while Max Verstappen and Red Bull Racing kept its powder dry.
Max Verstappen was only 11th at the end of the session, one spot up from team-mate Red Bull Racing having only completed medium-tyre runnng.
George Russell was third from Lando Norris while Daniel Ricciardo was just 0.026s back from his RB team-mate, Yuki Tsunoda in eight and ninth.
It was a busy start to a session that threatened a slight chance of rain later on.
All 10 teams have upgrades for this weekend, primarily with a view to ‘increase load’ to their respective machines.
That resulted in flo-vis paint up and down the field.
McLaren, Ferrari, and Red Bull Racing all had spells at the top of the tinesheets in the early moments as the order changed rapidly.
Though Formula 3 had been on track earlier for a two-part qualifying session, rain on Thursday ensured the track remained greasy initially before times tumbled quickly as both it gained rubber and drivers began pushing harder.
Pierre Gasly struck drama 15 minutes into the session when he was called back into the lane for a power unit issue in his Alpine.
The Frenchman had completed six laps but not logged a time as the squad pushed his A524 back into the shed for what amounted to a five-minutes delay.
Plenty of drivers were making mistakes; Nico Hulkenberg sent it long at Sainte Devote, Charles Leclerc bounced over the kerbs exiting the Swimming Pool, and Lewis Hamilton skated through the Nouvelle Chicane. Valtteri Bottas also found himself up the escape road at Sainte Devote.
Through it all, Leclerc emerged fastest after 20 minutes, his 1:13.343s proving three-tenths faster than Ferrari team-mate Carlos Sainz.
The field spread was reasonably significant however, evidenced by Yuki Tsunoda who rose to third with a 1:13.890s and bumped Lando Norris to fourth.
Having returned to the track following his earlier drama, Gasly was back in the lane as Alpine investigated a wastegate issue on his car.
With 33 minutes remaining, the team admitted it was unlikely the Frenchman would take any further part in the session.
At the halfway point, Leclerc was still fastest with a 1:13.132s, Lewis Hamilton having popped into second ahead of Verstappen as teams began to bolt on a set of soft compound tyres for their qualifying simulations.
Oscar Piastri was one such driver, climbing to the top of the timesheets with a 1:12.618s on the soft rubber.
It stood only briefly as George Russell logged a 1:12.295s to better the Australian by more than 0.3s. Hamilton too went faster than Piastri to move up to second, only seconds before the McLaren completed a second timed lap.
That was good enough to restore himself at the top of the timesheets, with a 1:12.198s after what was an aggressive lap.
Zhou Guanyu had a moment exiting Sainte Devote as his Sauber stepped sideways, glancing the wall on corner exit and as he gathered it back up.
He damaged the front wing endplate in the process, littering the circuit with debris and triggering a red flag.
Under the stoppage, Hamilton had pipped Piastri to sit fastest, 0.029s faster than Piastri had managed earlier.
Just 10 minutes remained when the session resumed, having been interrupted for a total of five minutes as marshals cleaned up after Zhou’s brush with the barrier.
Inside the final five minutes of the session, Sergio Perez reported he had a puncture.
The Mexican had been largely anonymous throughout the season, and sat 12th at the time of his dramae.
It was only one place back from team-mate Verstappen, the Dutchman having completed the session exclusively on the medium rubber.
The same explanation accounted for Perez’s lowly position, and that of Sainz, too, who was only one spot better off in 10th.
Gasly did get back out on track and was circulating as the chequered flag waved to end the opening hour of running.
It did so with Hamilton fastest from Piastri, then came Russell, Norris, Leclerc, Fernando Alonso, Lance Stroll, Yuki Tsunoda, and Daniel Ricciardo ahead of Sainz in 10th.
