'Fastest on Friday' Bottas tops incident-packed Portuguese GP practice
The Finn, who is 69 points adrift of Mercedes team-mate Lewis Hamilton in the title race, clocked a best lap in one minute and 17.940 seconds to beat Red Bull's Max Verstappen by six-tenths of a second.
The Dutchman was involved in a collision with Racing Point's Lance Stroll that halted the action and required both men to visit the stewards, Pierre Gasly's Alpha Tauri having suffered a fire that caused an earlier stoppage.
Hamilton, who is hunting a record 92nd career win on Sunday, wound up eighth, 1.5 seconds off the pace, but without producing a competitive lap in an interrupted session that was twice stopped by red flags.
McLaren's Lando Norris was third ahead of Charles Leclerc of Ferrari, Carlos Sainz in the second McLaren and four-time champion Sebastian Vettel in the second Ferrari.
Gasly, who was unhurt after jumping out of his fiery car, was seventh ahead of six-time champion Hamilton, Esteban Ocon of Renault and Alex Albon, in the second Red Bull.
On a bright and cool afternoon in southern Portugal, the undulating track provided little grip and many challenges throughout the day.
Temperatures fell steadily in the afternoon session with two red flags interrupting the action.
The action began with Sainz spinning twice during a Pirelli prototype tyres test, in which Bottas was quickest and several drivers had lap times deleted for exceeding track limits.
Once the 'normal' tyres were back in use, Leclerc went top before Ferrari team-mate Vettel spun at Turn 14 and pitted.
Predictably, Bottas regained the initiative before Gasly's Alpha Tauri burst into flames, forcing him to pull up.
He climbed out safely as the marshals moved in and the session was paused for 16 minutes.
After the re-start, Covid-19 returnee Stroll collided with Verstappen at Turn One, the Dutchman appearing to pay little heed to his rival's position as he went outside him.
Stroll's car was sent spinning into the gravel.
In a race, it might have been seen as a racing incident with blame for both men, but Verstappen was adamant, on team radio, that the Canadian driver required an eyesight test.
Video replays suggested that Verstappen may have thought Stroll was completing a flying lap and easing off to allow him to begin his own but Racing Point's team chief Otmar Schafnaeur said Stroll was doing two successive fast laps.
After another 15-minute delay to clear stranded cars and debris, the session resumed with eight minutes to go and everyone on softs for a rapid finale.