Sebastian Vettel kept Red Bull on top at Spa with a late flyer in a dry final Belgian Grand Prix practice session.
Fernando Alonso put Ferrari in second place, ahead of Mark Webber and an impressive run from Toro Rosso's Jean-Eric Vergne.
Much of the session had been conducted on Pirelli's hard compound, but with 15 minutes left on the clock the entire field pitted to bolt on the medium option.
Force India's Adrian Sutil was the first man to set a time on the new compound and instantly went quickest, but his time was soon eclipsed by Alonso's 1m48.432s.
That stood as the session's benchmark until the final two minutes, when Vettel found just over one tenth of a second to keep Red Bull in the ascendancy.
Felipe Massa and Jenson Button rounded out the top six, while Sauber's Esteban Gutierrez was a surprise seventh, fractions ahead of the lead Lotus of Romain Grosjean.
That meant no ostensible overnight cure to Mercedes' struggles, with Nico Rosberg ninth and Lewis Hamilton 12th, more than 0.7s down on Vettel.
As had been the case in Friday's opening practice, drivers had the early challenge of coping with a circuit that was damp in patches and completely dry in others.
That meant several moments of wild understeer or oversteer - the latter effect almost pitching Paul di Resta and Felipe Massa into the Turn 9 barriers early on - and a spin for Lewis Hamilton at the final chicane.
In such conditions it was Jean-Eric Vergne who was the early pacesetter, working his way down to a 1m49.479s on Pirelli's hard compound.
That put the Frenchman almost a quarter of a second clear and disrupted what had been an incredibly tight fight at the top of the timesheets, with the top seven at one stage covered by less than two tenths of a second.
Kimi Raikkonen had held top spot until Vergne's flyer, but struggled to translate his form on the medium compound and wound up 10th overall.
Bottas, who like Raikkonen had enjoyed early spells at the head of the field, sank to 14th.
Pos Driver Team/Car Time Gap Laps
1. Sebastian Vettel Red Bull-Renault 1m48.327s 16
2. Fernando Alonso Ferrari 1m48.432s +0.105s 14
3. Mark Webber Red Bull-Renault 1m48.533s +0.206s 16
4. Jean-Eric Vergne Toro Rosso-Ferrari 1m48.776s +0.449s 15
5. Felipe Massa Ferrari 1m48.788s +0.461s 15
6. Jenson Button McLaren-Mercedes 1m48.882s +0.555s 13
7. Esteban Gutierrez Sauber-Ferrari 1m48.930s +0.603s 18
8. Romain Grosjean Lotus-Renault 1m48.967s +0.640s 14
9. Nico Rosberg Mercedes 1m48.993s +0.666s 16
10. Kimi Raikkonen Lotus-Renault 1m49.008s +0.681s 17
11. Daniel Ricciardo Toro Rosso-Ferrari 1m49.035s +0.708s 15
12. Lewis Hamilton Mercedes 1m49.046s +0.719s 17
13. Adrian Sutil Force India-Mercedes 1m49.122s +0.795s 15
14. Valtteri Bottas Williams-Renault 1m49.177s +0.850s 15
15. Paul di Resta Force India-Mercedes 1m49.226s +0.899s 14
16. Sergio Perez McLaren-Mercedes 1m49.471s +1.144s 14
17. Nico Hulkenberg Sauber-Ferrari 1m49.478s +1.151s 19
18. Pastor Maldonado Williams-Renault 1m50.418s +2.091s 14
19. Charles Pic Caterham-Renault 1m51.416s +3.089s 15
20. Giedo van der Garde Caterham-Renault 1m51.800s +3.473s 18
21. Jules Bianchi Marussia-Cosworth 1m52.221s +3.894s 17
22. Max Chilton Marussia-Cosworth 1m53.507s +5.180s 18