Nick Heidfeld's test with Williams has been brought forward to Wednesday after BAR blocked plans for Anthony Davidson to drive their Formula One rivals' car.
German Heidfeld, who raced for Jordan this year, is hoping to impress the BMW-powered team enough to land a drive for 2005. He had been due to test from December 9.
Briton Davidson, the highly-rated BAR reserve driver, had wanted to test for Williams at Jerez in southern Spain this week but the arrangement fell through due to contractual problems.
Williams have the most coveted vacancy in Formula One, with the team still to announce a partner for Australian Mark Webber.
Brazilian Antonio Pizzonia, who has been a regular test driver for Williams this season after losing his place as Webber's Jaguar team mate in 2003, remains the favourite to secure the seat.
Williams will have an all-new line-up in 2005 after Germany's Ralf Schumacher and Colombian Juan Pablo Montoya left for Toyota and McLaren respectively.
Webber and Pizzonia were the only drivers testing at Jerez on Tuesday but all the other teams, except Minardi, are due at the circuit this week.
With Heidfeld in the Williams, Dutch drivers Christijan Albers and Robert Doornbos will be testing for Jordan in the team's first session since the end of the season in October.