The sport returned from its four-week summer break with much anticipation ahead of one of the most popular race weekends of the year – with five drivers still in contention for the title with seven races to go.
Alonso set the benchmark half-way through the 90-minute session before the rain worsened and all but ended any competitive running until, with 10 minutes to go, blue skies finally replaced the dark clouds.
Changeable weather is almost a guarantee at Spa-Francorchamps and Friday was no different; the circuit remained treacherous, catching out Felipe Massa, Nico HÃƒÂ¼lkenberg, and Heikki Kovalainen.
But as track conditions improved, Alonso went quicker again in his Ferrari to record a time of 2:00.797 – eight tenths quicker than McLarenÃ¢â‚¬â„¢s Lewis Hamilton.
Robert Kubica, featuring RenaultÃ¢â‚¬â„¢s F-duct for the first time, nicked third-quickest for Renault from Sebastian Vettel with minutes to go, just 1.2 seconds slower than Alonso. He was followed by Red BullÃ¢â‚¬â„¢s Sebastian Vettel, and Force IndiaÃ¢â‚¬â„¢s Adrian Sutil fifth.
Jenson Button just pipped Mark Webber to finish sixth-quickest for McLaren – 2.1 seconds slower than Alonso from just six completed laps.
Sauber rookie Kamui Kobayashi was 2.6 seconds off AlonsoÃ¢â‚¬â„¢s time, just ahead of anniversary boy Rubens Barrichello with ninth-quickest at his 300th grand prix, and BarrichelloÃ¢â‚¬â„¢s former team-mate Michael Schumacher, who rounded out the top 10.
Virgin Racing got one over on newcomer rivals Lotus Racing as both Timo Glock and Lucas di Grassi outpaced Jarno Trulli, while Bruno Senna finished ahead of Heikki Kovalainen for Hispania with Sakon Yamamoto once again bringing up the rear.
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BIOGRAPHY: Cesc Fabregas