Nakajima finished 15th for Williams in the 2008 drivers championship

Williams' Japanese driver Kazuki Nakajima set the fastest time of the week on the final day of Formula 1 testing in Jerez.

Nakajima, 24, clocked a best time of one minute 17.494, while Heikki Kovalainen showed that McLaren will be competitive with a time of 1:17.946.

The previous best had been Jenson Button's 1:17.844 for Brawn GP.

The four-day test was the last before the 2009 season gets underway in Australia on 29 March.

Testing is now banned until the end of the year, apart from a few days of straight-line aerodynamic testing.

The British-based Williams team finished eighth in the constructors' world championship last year but Nakajima and fellow driver Nico Rosberg will be boosted by the car's performance in testing.

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"We have put over 8,000 km on the FW31 (car) in the last two months and, while there are always areas that require attention, reliability has been good," said Williams technical director Sam Michael.

Nakajima finished in the points five times last season, scoring nine in total, but came 15th in the overall standings.

F1 champion Lewis Hamilton was a couple of seconds off the pace in testing with a best lap of 1:19.121 set on Tuesday.