AlphaTauri racer Gasly had used the cooler conditions as dusk arrived over the Sakhir circuit late into the four-hour afternoon session to bolt to the top of the timing screens.
Equipped with the softest C5 compound Pirelli tyre, Gasly overcame the gusty Gulf air to post a 1m33.902s to sit half a second clear at the top over the two Ferraris.
But any late challenges were denied by Perez when the Red Bull driver caused the second red flag of the day with just eight minutes remaining on the clock.
As virtual safety car conditions were imposed as part of a customary run through late on, at reduced speeds the Mexican lost the rear of his RB18 at the exit of Turn 8.
The car then appeared to roll backwards slowly before the rear axle dropped off the edge of the run-off and into a gravel trap, where Perez became beached and the session was ended.
It marked a contrast to the start of the four-hour afternoon run, which took its time to kick into action as initially none of the 10 cars headed out on track.
It was a full nine minutes before the session burst into life, with Haas the first to pull the trigger as the team attempted to play catch up following a morning lost to set-up while it recovered from the delayed arrival of its freight.
Test driver Pietro Fittipaldi, who briefly steps into the vacancy created by Nikita Mazepin before Kevin Magnussen returns full-time, sported the car’s revised red and white livery.
The Brazilian, who raced at the track in 2020 in place of the injured Romain Grosjean, only completed the one lap before returning to the pits and leaving Lando Norris out alone.
Despite the clear road, however, it looked as though the 1m34.531s effort set by Charles Leclerc to top the morning session would have remained fastest all day.
That was until the run delivered by the Ferrari driver on the yellow-walled Pirellis was eventually deposed by Gasly towards the end of his 103 laps and then by team-mate Carlos Sainz Jr.
The Spaniard, replacing Leclerc in the car for the afternoon, turned in 52 laps as he ran to a 1m34.359s.
Leclerc’s earlier effort was strong enough to secure third overall as Lance Stroll ran to fourth.
The Aston Martin driver had been the cause of the earlier red flag, waved 45 minutes after the lunch break, when the aerodynamic rakes on his car worked themselves loose.
A brace of the devices were mounted just in front of the rear tyres but as Stroll picked up the pace and clipped the kerbs, they slipped from position and began to rub the tyres.
After the initial smoke from the contact, the right-rear grille detached fully down the back straight and forced marshals to clear the track.
Stroll also created some late action with the Alpine of Fernando Alonso as the two ran wheel-to-wheel through the opening sector.
Williams’ Alexander Albon snared the fifth fastest time of the day, despite not improving from his morning effort, as he ran for a total of 104 laps across both sessions.
However, Albon had reported a steering issue that truncated running while the hydraulics were checked.
Similarly, Norris had to grapple with morning brake issues as he clocked sixth after filling in for an unwell Daniel Ricciardo aboard the McLaren MCL36.
Valtteri Bottas turned in 66 laps for Alfa Romeo as he ran to seventh ahead of Sebastian Vettel and George Russell, who complained of the car bottoming out on the main straight.
Mercedes, with its W13 sporting radical new minimalist sidepods, missed the first hour of the afternoon as it carried out maintenance on the car.
Perez, meanwhile, completed the top 10 over Lewis Hamilton as Alonso pipped Alpine team-mate Esteban Ocon.
Alfa Romeo rookie Guanyu Zhou was kept off the bottom of the timing screens by Fittipaldi, who ran the prototype tyre to a 1m37.422s on his return to the cockpit – 3.5s adrift of Gasly.
Bahrain F1 pre-season test - Day 1 results:
| Cla | Driver | Chassis | Laps | Time | Gap | |
|---|---|---|---|---|---|---|
| 1 |  Pierre Gasly |  | AlphaTauri | 103 | 1'33.902 | |
| 2 |  Carlos Sainz Jr. |  | Ferrari | 52 | 1'34.359 | 0.457 |
| 3 |  Charles Leclerc |  | Ferrari | 64 | 1'34.531 | 0.629 |
| 4 |  Lance Stroll |  | Aston Martin | 50 | 1'34.736 | 0.834 |
| 5 |  Alex Albon |  | Williams | 104 | 1'35.070 | 1.168 |
| 6 |  Lando Norris |  | McLaren | 50 | 1'35.356 | 1.454 |
| 7 |  Valtteri Bottas |  | Alfa Romeo | 66 | 1'35.495 | 1.593 |
| 8 |  Sebastian Vettel |  | Aston Martin | 39 | 1'35.706 | 1.804 |
| 9 | George Russell |  | Mercedes | 60 | 1'35.941 | 2.039 |
| 10 |  Sergio Perez | | Red Bull | 138 | 1'35.977 | 2.075 |
| 11 | Lewis Hamilton |  | Mercedes | 62 | 1'36.365 | 2.463 |
| 12 | Fernando Alonso |  | Alpine | 24 | 1'36.745 | 2.843 |
| 13 | Esteban Ocon |  | Alpine | 42 | 1'36.768 | 2.866 |
| 14 |  Guanyu Zhou | | Alfa Romeo | 54 | 1'37.164 | 3.262 |
| 15 |  Pietro Fittipaldi |  | Haas | 47 | 1'37.422 | 3.520 |
