Scattered showers marred Nick Heidfeld's test drive for Williams in southern Spain.
The two drivers are vying for a place alongside Australian Mark Webber at BMW-Williams for next season.
Nick Heidfeld joined the Mercedes Formula One team as test and reserve driver on Thursday, completing a trio of Germans in a line-up led by Michael Schumacher and Nico Rosberg.
Nick Heidfeld's test with Williams, due on December 9, has been brought forward to Wednesday.
Williams hopeful Nick Heidfeld was fastest at the start of what could be a decisive five-day test programme with the Formula One team in southern Spain on Monday.
Williams' Nick Heidfeld will miss next week's Brazilian Grand Prix after a bicycle accident at the weekend
McLaren's Lewis Hamilton took his first Formula One victory on Sunday, keeping his cool in a chaotic Canadian Grand Prix to take the outright lead in the championship.
Sebastian Vettel powered to victory in the Malaysian Grand Prix on Sunday, the German registering his second pole-to-flag win in as many races to open his Formula One title defence.
BMW's Nick Heidfeld believes F1's return to Europe could force re-writing of the early-season form book
The 31-year-old German fell 30 points behind race-winner Hamilton after a collision with the Briton left him struggling to finish fourth in Monza last time out, one of several races where Vettel has frittered away a position of authority.
