The hour-long final practice session assumed greater significance after Friday's two 90-minute sessions were virtually washed out by incessant heavy rain.
Under blue skies fellow title hopeful Kimi Raikkonen put his Lotus second behind the Ferrari - Alonso's time was a minute 48.542, over a minute quicker than the fastest time in second practice and 0.141s quicker than the Finn - with Sergio Perez and Kamui Kobayashi following as Sauber impressed.
Jenson Button was half a second off the pace in fifth while fellow Briton Lewis Hamilton, McLaren's biggest hope for the title, was a further half a second down in 12th.
Red Bull duo Mark Webber and Sebastian Vettel were seventh and ninth. Australian Webber will have a five-place grid penalty for the race after a gearbox change on Friday.
There were problems for Nico Rosberg after the German's Mercedes was stranded with what seemed to be a transmission problem. His team-mate Michael Schumacher, competing in his 300th grand prix this weekend, was down in 13th.
The teams adopted different approaches for the majority of the session, with some looking for qualifying set-up on the softer medium-compound tyres and others on long fuel runs on the hard rubber.
Around the halfway point Alonso, who was already top of the order with his hard-compound time, improved further on the softer tyres before Perez and Kobayashi put the Saubers first and second during a quiet spell on track 10 minutes later.
Romain Grosjean, who finished between the Red Bulls in eighth, went top in his Lotus briefly before Webber set a new benchmark on hard tyres.
Sauber returned to the top two positions late on before former world champions Alonso and Raikkonen's final charge.
Lotus' plan to test what they call 'The Device' - intended to boost their straight-line speed - was vetoed after Friday's washout while Alonso's claim that Ferrari are the slowest of the top teams seems to be mere psychological wordplay.