Max Verstappen completes practice sweep in Spain ahead of qualifying
The championship leader's best lap of one minute 13.664 seconds on the soft tyres was 0.250 quicker than the Mexican's best effort at Barcelona's Circuit de Catalunya.
Seven-time world champion Lewis Hamilton (38) was third fastest for Mercedes, 0.408 off the pace, with Spaniards Carlos Sainz (28) and Fernando Alonso (41) fourth and fifth for Ferrari and Aston Martin.
Mercedes' George Russell (25) was sixth and Ferrari's Charles Leclerc (25), who reported a strange sound from the clutch while doing a practice start, seventh.
Hamilton had said on Friday, when he was 12th and 11th in the two sessions, that he hoped overnight changes would help him get into the top 10 with a car that Mercedes have re-designed after a disappointing start to the season.
The session was red-flagged after nine minutes when Williams' rookie Logan Sargeant (22) crashed at the final corner before it started to rain.
McLaren's Lando Norris (23) ventured out with 28 minutes to go to test the conditions after a long period with no action.
The conditions then improved, allowing drivers to run on slicks again for the final few minutes although without testing the limits.
Hamilton completed only nine laps, spending much of the session out of the car but jumping back in again at the end as the weather improved.
"I don't feel comfortable pushing in these conditions. I think it's a waste," Verstappen had told his team over the radio.