On a hot Friday morning Haas' Kevin Magnussen led the early running with a first representative time, a 1m33.710s on Pirelli's mid-range C3 compound.
Sainz then grabbed the lead with a 1m32.653s effort, which he later improved to a 1m32.486s in his C3-shod Ferrari.
It meant the Spaniard had improved on Max Verstappen's Thursday benchmark by four tenths, with Ferrari continuing to head the top speed charts.
Sainz' compatriot Fernando Alonso soon joined him at the front after the Aston Martin driver posted a 1m33.278s and then a 1m33.182s on the same compound.
Just before the half-way mark of the 4h15m morning session, Alfa Romeo's Zhou Guanyu split the Spaniards by going second with a 1m33.170s lap.
After disappearing into the garage for an hour to work on the front-end, Alonso returned to reclaim the spot with a 1m32.969s on C2s.
Zhou's morning was a mixed bag, too, with the Chinese driver disappearing for a lengthy spell in the garage which restricted him to 43 laps.
Newcomer Sargeant was the last of the 10 morning runners to get a time on the board as the American continued acclimatising to the FW45.
In the final hour he gradually moved up to second with a 1m32.968s and then set a late 1m32.549s to get within 0.063s of Sainz' benchmark.
Behind Alonso and Zhou, Magnussen was next up in fifth. The Haas man was the last driver to stay within a second of the leader, followed by Alpine's Esteban Ocon.
Friday morning marked the first appearance for Sergio Perez, as Red Bull was the only team not to split Thursday between its two drivers.
On his 2023 test debut Perez worked through plenty of test items on a productive morning, just like team-mate Verstappen on Thursday. The Mexican completed 76 laps, the best of which kept him in seventh.
Lewis Hamilton had a low-key session for Mercedes, remaining in eighth with team boss Toto Wolff admitting to F1 TV its W14 car "was out of balance" in the morning heat, circumstances which are not representative for the Bahrain Grand Prix's night-time qualifying and race.
The seven-time world champion did return the garage for some minor repairs after being one of several cars to shed pieces of bodywork.
Far behind the other runners, Lando Norris was ninth for McLaren, appearing to struggle with the handling of a bucking MCL60.
Yuki Tsunoda was Friday morning's productivity king, logging a mammoth 85 laps as he finished bottom on the timesheets.
The trouble-free session allowed teams to get plenty of mileage in and help Pirelli with development work on C3 prototypes.
The only stoppage occurred at the end; a virtual safety car followed by a brief red flag for a systems test by race control.
That then led to a pair of practice starts to bring the session to a close. On the second start Norris provided some minor excitement by moving away slowly, Sainz having to swerve to avoid hitting his former McLaren team-mate.
After a one-hour lunch break testing resumes at 15:15 local Bahrain time.
Morning test times
| Pos | Driver | Car | Time | Gap | Laps |
|---|---|---|---|---|---|
| 1 | Carlos Sainz | Ferrari SF-23 | 1'32.486 | 70 | |
| 2 | Logan Sargeant | Williams FW45 | 1'32.549 | 0.063 | 74 |
| 3 | Fernando Alonso | Aston Martin AMR23 | 1'32.969 | 0.483 | 64 |
| 4 | Zhou Guanyu | Alfa Romeo C43 | 1'33.170 | 0.684 | 43 |
| 5 | Kevin Magnussen | Haas VF-23 | 1'33.442 | 0.956 | 67 |
| 6 | Esteban Ocon | Alpine A523 | 1'33.490 | 1.004 | 49 |
| 7 | Sergio Perez | Red Bull RB19 | 1'33.751 | 1.265 | 76 |
| 8 | Lewis Hamilton | Mercedes F1 W14 E Performance | 1'33.954 | 1.468 | 72 |
| 9 | Lando Norris | McLaren MCL60 | 1'35.522 | 3.036 | 65 |
| 10 | Yuki Tsunoda | AlphaTauri AT04 | 1'35.708 | 3.222 | 85 |
