Belgian GP: Sebastian Vettel keeps Red Bull on top in practice three

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