As a dim afternoon set in at Jerez, Jaime Alguersuari looked set to finish the third day of F1's second group test with the quickest time.
At the wheel of the Toro Rosso, the Spaniard - whose teammate Sebastien Buemi was quick a day earlier - managed the best lap time of the entire test before the rains began to fall in mid-morning.
Sauber's 2010 car continued to impress, with Pedro de la Rosa setting the next best dry time, ahead of Adrian Sutil in the Force India.
Spaniard de la Rosa, however, also caused a red flag, with the C29 needing to be returned to the pits on a flat-bed truck following a technical fault.
Also testing on Friday, the penultimate day of the four-day Jerez test, are (in order of lap time) Felipe Massa, Sebastian Vettel, Vitaly Petrov, Nico Rosberg, Rubens Barrichello and Rubens Barrichello.
Virgin's pit garage doors remained pulled down throughout most of Friday, with the new team still waiting for new parts to arrive from Britain, following the front wing failure suffered by Timo Glock on Thursday.
A spokeswoman said the plan is for some mid-afternoon running with Lucas di Grassi.
Rosberg and Barrichello both interrupted proceedings with minor offs, as predictions of rainy weather in the Andalusian region proved correct.