Alonso's timesheet-topping run around the seven-kilometre track, the longest on the F1 calendar, was ultimately seven seconds slower than the quickest lap set in practice a year ago by the Spaniard in his Ferrari as showers dominated the 90-minute session.
The 32-year-old, who is in need of a strong result this weekend to close the 39-point gap to championship leader Sebastian Vettel of Red Bull, set a lap of one minute 55.198 seconds.
Force India's Paul Di Resta, himself desperate for points after not scoring in the last two grands prix, finished just 0.026secs adrift of Alonso as the field was effectively turned on its head.
At the bottom of the standings, for instance, were Lotus duo Kimi Raikkonen and Romain Grosjean, the latter completing 15 laps, the third highest total of the morning.
Raikkonen, seemingly fit again after missing his media duties on Thursday due to illness, was over four seconds down on Alonso, with Grosjean nearly eight seconds off the pace.
Di Resta's team-mate Adrian Sutil was third-fastest, followed by McLaren's Sergio Perez and Nico Rosberg in his Mercedes, with Vettel sixth, 0.438secs down.
Sauber duo Esteban Gutierrez and Nico Hulkenberg both managed to finish within a second of Alonso, the only other drivers to do so.
As for the rest of the field, led by Toro Rosso's Daniel Ricciardo, they were some way off the pace, the Australian 1.5secs adrift.
Down in 14th and 15th positions were Jenson Button in his McLaren and Mercedes' Lewis Hamilton. The latter has claimed pole in the last three races and is fresh from his victory in Hungary four weeks ago, but both British drivers were two seconds down.
Marussia's Max Chilton was 18th and 3.4secs behind, one place ahead of Mark Webber in his Red Bull.