Red Bull's Max Verstappen lapped fastest in final practice for the Portuguese Grand Prix on Saturday, with seven-times world champion Lewis Hamilton second for Mercedes.
Hamilton leads the standings after two races, a point clear of his Dutch rival thanks to a fastest lap at the previous race in Imola.
Both have won and finished second once.
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"It's impossible to say now who the favourite is for qualifying. I think they are absolutely equal pace and it will just come down to the driver," commented 2016 world champion Nico Rosberg, Hamilton's former team mate.
It was the first time this weekend that Red Bull had led a practice session after Bottas and Hamilton were quickest in Friday's two.
Mexican Sergio Perez was fourth fastest for Red Bull with Esteban Ocon fifth for Renault-owned Alpine, who appear to have picked up pace this weekend.
The Ferrari pairing of Charles Leclerc and Carlos Sainz were sixth and seventh with McLaren's Lando Norris, who spun, eighth on the timesheets.
The virtual safety car was deployed during the session after a track advertising hoarding blew over in gusty conditions and had to be retrieved. (Reporting by Alan Baldwin in London, editing by Ed Osmond)
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