Lewis Hamilton could only manage the fourth-fastest time as Daniil Kvyat recovered from fuel system issues to top the Singapore Grand Prix second practice standings.
Kvyat came back from an 18th-place finish in the first session to lead the standings with a time of 1:46.142, just 39 hundredths ahead of Ferrari's Kimi Raikkonen.
Both Hamilton and Mercedes teammate Nico Rosberg had a poor race, with the British driver 0.337s off the pace and the German dropping from first in the initial practice to seventh in the follow-up.
Kvyat's Red Bull teammate Daniel Ricciardo came third as the top three drivers were separated by only 0.114s, while Force India's Sergio Perez was sixth and McLaren's Fernando Alonso finished in eighth behind Rosberg.
Manor Marussia's difficult start to the weekend continued as Alexander Rossi's crash at Turn 18 of FP1 was followed by teammate Will Stevens spinning into the barrier at Turn 11 early on in the second practice.
1. Daniil Kvyat (Red Bull)
2. Kimi Raikkonen (Ferrari)
3. Daniel Ricciardo (Red Bull)
4. Lewis Hamilton (Mercedes)
5. Sebastian Vettel (Ferrari)
6. Sergio Perez (Force India)
7. Nico Rosberg (Mercedes)
8. Fernando Alonso (McLaren)
9. Nico Hulkenberg (Force India)
10. Max Verstappen (Toro Rosso)
11. Felipe Massa (Williams)
12. Felipe Nasr (Sauber)
13. Marcus Ericsson (Sauber)
14. Jenson Button (McLaren)
15. Carlos Sainz (Toro Rosso)
16. Romain Grosjean (Lotus)
17. Valtteri Bottas (Williams)
18. Pastor Maldonado (Lotus)
19. Alexander Rossi (Marussia)
20. Will Stevens (Marussia)
