Sebastian Vettel proved to be the driver to beat at Spa on Friday afternoon but there were also concerns as he suffered a right rear tyre failure.
With the sun shining down on the Belgian circuit, the drivers were quickly down to business on the hard tyres with Vettel leading the way with a 1:50.555s.
A swap to the option medium tyres saw Mark Webber take over at the front only to lose out to Vettel when the German also made the change. A 1:49.331 for the reigning World Champ.
However, it wasn't all smooth sailing for Vettel as his session was halted with 20 minutes remaining when he suffered a rear right tyre failure.
The Red Bull racer, who was seven laps into his hard tyre run, immediately realised something was wrong and slowed down, limping back to the pits. But as his car suffered floor damage it was game over.
Vettel wasn't the session's only casualty as minutes after his incident Giedo van der Garde slammed into the barriers after the Paul Frere curve when he lost control of his CT03.
Back at the front Romain Grosjean was third for Lotus, finishing ahead of Felipe Massa, Jean-Eric Vergne and Kimi Raikkonen. The quartet were the only drivers to get within a second of Vettel's P1 time.
Fernando Alonso was seventh after leaving his medium tyre run to late in the session while Sergio Perez brought his McLaren home in eighth place ahead of Nico Rosberg and Paul di Resta.
01 Sebastian Vettel Red Bull 1:49.331 22 laps
2 Mark Webber Red Bull 1:49.390 0.059 34 laps
3 Romain Grosjean Lotus 1:50.149 0.818 34 laps
4 Felipe Massa Ferrari 1:50.164 0.833 27 laps
5 Jean-Eric Vergne Toro Rosso 1:50.253 0.922 28 laps
6 Kimi Raikkonen Lotus 1:50.318 0.987 33 laps
7 Fernando Alonso Ferrari 1:50.510 1.179 21 laps
8 Sergio Perez McLaren 1:50.536 1.205 27 laps
9 Nico Rosberg Mercedes 1:50.601 1.270 33 laps
10 Paul di Resta Force India 1:50.611 1.280 27 laps
11 Adrian Sutil Force India 1:50.629 1.298 30 laps
12 Lewis Hamilton Mercedes 1:50.751 1.420 27 laps
13 Nico Hulkenberg Sauber 1:50.972 1.641 33 laps
14 Pastor Maldonado Williams 1:50.991 1.660 28 laps
15 Jenson Button McLaren 1:51.195 1.864 28 laps
16 Daniel Ricciardo Toro Rosso 1:51.447 2.116 26 laps
17 Valtteri Bottas Williams 1:51.568 2.237 28 laps
18 Esteban Gutierrez Sauber 1:51.644 2.313 26 laps
19 Giedo van der Garde Caterham 1:53.157 3.826 21 laps
20 Charles Pic Caterham 1:53.251 3.920 29 laps
21 Jules Bianchi Marussia 1:53.482 4.151 28 laps
22 Max Chilton Marussia 1:54.418 5.087 12 laps
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