Zhou Guanyu triggered a testing surprise with a late glory lap to top the times for Alfa Romeo.
The Chinese driver posted a lap of one minute, 31.610secs to finish just 0,04s faster than Red Bull's Max Verstappen, who again enjoyed a trouble-free afternoon.
But the headline from the session was the hydraulic failure suffered by George Russell that cut Mercedes' run plan short.
READ MORE: Mercedes confirm Russell hydraulic failure after red flag
After a largely comfortable initial one and a half days of the Bahrain pre-season test, teams began to experience troubles late on the second day, with Zhou having to be pushed back to the garage after failing to get away from the practice start area in the final 15 minutes.
Logan Sargeant simultaneously experienced what seemed to be a hybrid system issue at Williams, though the calmness of the situation outside the garage and the green light present on the roll hoop suggest it may have been a procedural dress rehearsal.
It transpired that it was a dress rehearsal, during which an issue was discovered on the FW45.
Zhou's lap in the final hour was on tyres two steps softer than Verstappen, underlining the unrepresentative nature of the timesheets.
Fernando Alonso again finished at the top, third fastest for Aston Martin with 130 laps under his belt across the day.
AlphaTauri experienced its best session yet with Nyck de Vries fourth quickest with over 70 laps, ahead of Nico Hulkenberg who showed promise for Haas.
Sainz stayed in sixth following his morning effort, with Ferrari's afternoon duties given to Charles Leclerc - both Scuderia drivers sandwiching Sargeant who impressed on his second day.
READ MORE: Wolff reveals 'mentally difficult' Mercedes challenge
Oscar Piastri notched 74 laps in ninth for McLaren, with Lando Norris down in 16th after his morning run, with Pierre Gasly rounding out the top 10 for Alpine.
George Russell was the slowest of the afternoon runners, with his hydraulic issue forcing him to stop in the Mercedes with just over half the session gone.
F1 Bahrain pre-season test second day, complete standings
1. Zhou Guanyu [Alfa Romeo] - 1:31.610 - 133 laps
2. Max Verstappen [Red Bull] - +0.040secs - 47 laps
3. Fernando Alonso [Aston Martin] - +0.595s - 130 laps
4. Nyck de Vries [AlphaTauri] - +0.612s - 74 laps
5. Nico Hulkenberg [Haas] - +0.856s - 68 laps
6. Carlos Sainz [Ferrari] - +0.876s - 70 laps
7. Logan Sargeant [Williams] - +0.939s - 154 laps
8. Charles Leclerc [Ferrari] - +1.115s - 68 laps
9. Oscar Piastri [McLaren] - +1.565s - 74 laps
10. Pierre Gasly [Alpine] - +1.576s - 59 laps
11. Kevin Magnussen [Haas] - +1.832s - 67 laps
12. Esteban Ocon [Alpine] - +1.880s - 49 laps
13. George Russell [Mercedes] - +2.044s - 26 laps
14. Sergio Perez [Red Bull] - +2.141s - 76 laps
15. Lewis Hamilton [Mercedes] - +2.344s - 72 laps
16. Lando Norris [McLaren] - +3.912s - 65 laps
17. Yuki Tsunoda [AlphaTauri] - +4.098s - 85 laps