The reigning world champion completed just 37 laps in the RB7 as the Red Bull mechanics opened the car to carry out precautionary checks, but a stunning late time of 1:24.374 was 1.111s quicker than AlonsoÃ¢â‚¬â„¢s best time.
The test started on a damp track after overnight rain but it gradually dried out across the day allowing the teams to test out PirelliÃ¢â‚¬â„¢s intermediate tyres until a dry line formed around lunchtime.
The times tumbled as Toro RossoÃ¢â‚¬â„¢s Jaime Alguersuari, and then McLarenÃ¢â‚¬â„¢s Jenson Button went quickest on slicks, before Alonso trumped both of them, again looking imperious as the Ferrari 150th Italia completed over 100 laps without incident.
Many teams opted to focus on longer runs in the afternoon session as conditions improved, and that was when some of the quickest times came.
Alguersuari finished third quickest with a time of 1:25.641 in the STR6, while ButtonÃ¢â‚¬â„¢s time of 1:26.365 was enough for fifth quickest behind SauberÃ¢â‚¬â„¢s Kamui Kobayasho.
Paul di Resta was sixth in the VJM04 for Force India, just two tenths slower than the McLaren with the same engine despite only taking to the track late in the day as the team waiting on parts to arrive at the circuit.
Rubens Barrichello was seventh quickest for Williams without KERS with a time of 1:26.912, after earlier leading the standings in damp conditions before an engine issue sidelined the Brazilian who was quickest across testing in Jerez last week.
Michael Schumacher concentrated on race simulation in the afternoon as he completed a race distance including pit stops on his way to 90 laps and eighth quickest with a time of 1:27.512.
Narain Karthikeyan completed a productive 116 laps for Hispania as the Spanish team returned to testing after missing last weekÃ¢â‚¬â„¢s run-out in Jerez. The Indian driver got to experience the different Pirelli compounds as well as gaining mileage in the car with a quickest time of 1:28.393.
Heikki Kovalainen completed 58 laps across 10 runs throughout the day as the Team Lotus ran through a number of compound choices with a heavy fuel load before a gearbox issue prevented the team from going for a race distance. Kovalainen set his quickest time of 1:30.065 on lap 37, and hopes to be able to go for a race distance on Saturday.
JÃƒÂ©rÃƒÂ´me dÃ¢â‚¬â„¢Ambrosio also completed 116 laps for Virgin Racing in the MVR-02 with a full race distance with the new car as he achieved a time of 1:30.950.
Renault suffered a torrid day as a complicated KERS problem sidelined the R31 for much of the day.
The team opted to run both drivers to test a new traffic light pit stop system, with Nick HeidfeldÃ¢â‚¬â€standing for the injured Robert KubicaÃ¢â‚¬â€running in the morning in the wet conditions, while Vitaly Petrov was able to run in the final 40 minutes of the day.
HeidfeldÃ¢â‚¬â„¢s fastest lap was 20 seconds slower than Vettel and was indicative of the difference in track conditions in the morning rather than the carÃ¢â‚¬â„¢s performance as the German completed just 27 laps.
PetrovÃ¢â‚¬â„¢s afternoon was cut short, and his quickest time was also way off the pace from just 20 laps, but the team is confident that the KERS problem has been resolved and they will be on the pace in the remaining three days of the test, with both now drivers expected to get some running on Saturday.
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BIOGRAPHY: Eric Bailly