Nicholas Latifi took advantage of a drying track on Saturday to top the times for Williams ahead of Ferrari's Charles Leclerc in a rain-hit third and final practice for Sunday's Hungarian Grand Prix.
The 27-year-old Canadian claimed an unlikely first session-topping position of his career with a best lap in one minute and 41.480 seconds to outpace Leclerc by 0.661 seconds, with his Williams team-mate Alex Albon third, nine-tenths adrift.
World champion and series leader Max Verstappen was fourth for Red Bull ahead of Mercedes' George Russell and Fernando Alonso of Alpine.
Carlos Sainz was seventh in the second Ferrari ahead of Lando Norris, retirement-bound four-time world champion Sebastian Vettel, who crashed in the closing minutes, and Kevin Magnussen of Haas.
Lewis Hamilton was 11th for Mercedes after a difficult session in which both he and Russell struggled to generate any heat in their tyres while Sergio Perez also had problems and wound up 20th in the second Red Bull.
After a heavy downpour during the morning before the session began, Leclerc was the first man to venture out, followed by Sainz. The Spaniard was initially quickest before Leclerc went top in 1:43.364, an advantage of nine-tenths ahead of his team-mate.
Last year's winner Esteban Ocon and his Alpine teammate Alonso, both of whom claimed their maiden Formula One wins at the Hungaroring, were also quick to join the fray.
By the 20-minute mark, the order was unchanged at the top with Vettel third ahead of Aston Martin team-mate Lance Stroll, both enjoying something extra supplied by their new rear wing.
Most drivers reported a lot of standing water around the circuit, which explained why Hamilton and the two Red Bull drivers delayed their entry until the half-hour mark.
After 45 minutes, with the rain easing off, Russell had gone out and returned unhappy.
Both Leclerc and Pierre Gasly were among the drivers to survive full spins in the treacherous conditions, but Vettel found more pace to climb to second and split the Ferraris.
Twice a winner in Hungary, Vettel seemed to be relishing the conditions as did his long-time rival Alonso who improved to within three-tenths of Leclerc to take second before the German driver crashed after 51 minutes.
Vettel lost control at Turn 10 and slid backwards into the barriers, his collision prompting a red flag as he came to a halt in a gravel trap. He was cross, but unhurt.
The session restarted with four minutes remaining.
"It's like driving on ice," said Alfa Romeo's Zhou Guanyu.