Red Bull rolled out an updated version of its RB18 on the final day of pre-season testing that helped propel Sergio Perez to the fastest time in Bahrain this week.
The car sported a more sculpted sidepod and new floor and Perez wasted little time in showing off its potential, initially trading quickest laps with Alfa Romeo's Guanyu Zhou before blasting in a time of one minute 33.105secs on the C4 softs.
That lap proved to be a tenth-of-a-second faster than the leading time from Haas' Kevin Magnussen posted in his additional hour of running late on Friday.
Perez concluded his final outing in the car prior to next Friday's practice sessions at the Bahrain International Circuit by finishing 0.854s clear of Zhou who had enjoyed qualifying simulation runs early in the four-hour session.
Perez and Zhou were the only two drivers who popped in fast times as the other eight on track opted for longer running and fine-tuning of their cars.
AlphaTauri's Pierre Gasly was third quickest overall, albeit 1.7s behind Perez, and completing a morning high 91 laps.
Gasly was followed by Ferrari's Carlos Sainz in his Ferrari, Alpine's Fernando Alonso and McLaren's Lando Norris, running for a third consecutive day due to Daniel Ricciardo contracting Covid-19.
Wiliams' Nicholas Latifi enjoyed a trouble-free morning in contrast to what unfolded on Friday when his car caught fire, with the Canadian 2.5s behind Perez.
Aston Martin's Lance Stroll, Lewis Hamilton in his Mercedes and Magnussen completed the top 10, with the seven-time champion chalking up 78 laps on high-fuel running, with his best a 1:36.217s.
The session concluded with the FIA testing its start procedures, orchestrating a standing start involving Gasly, on 'pole position', followed by Perez, Stroll and Zhou.
Bahrain pre-season testing final day morning session times
1. Sergio Perez [Red Bull] 1:33.105 
2. Guanyu Zhou [Alfa Romeo] 1:33.959 
3. Pierre Gasly [AlphaTauri] 1:34.865 
4. Carlos Sainz [Ferrari] 1:34.905 
5. Fernando Alonso [Alpine] 1:35.328 
6. Lando Norris [McLaren] 1:35.504 
7. Nicholas Latifi [Williams] 1:35.634 
8. Lance Stroll [Aston Martin] 1:36.029 
9. Lewis Hamilton [Mercedes] 1:36.217 
10. Kevin Magnussen [Haas] 1:38.616