Spielberg bei Knittelfeld: Williams driver `c was fastest in the third and final practice here Saturday ahead of Sunday`s Austrian Grand Prix, setting up a real duel with Mercedes.
The Finn set a best time of one minute and 09.848 seconds on the 4.3-kilometre Red Bull Ring track ahead of Mercedes`s Lewis Hamilton at 0.050 seconds, and Williams teammate Felipe Massa at 0.053.
Hamilton`s Mercedes colleague and current championship leader Nico Rosberg also dipped below the 1min 10sec mark, just behind Toro Rosso`s Daniil Kvyat.
Fernando Alonso of Ferrari was sixth, ahead of Canadian Grand Prix winner Daniel Ricciardo of Red Bull and Kevin Magnussen of McLaren.
Ahead of the final qualifying round Saturday afternoon, Williams sent out a clear message that they can threaten the dominant Mercedes for a possible win at this first Austrian Grand Prix in 11 years.
Hamilton and Rosberg dominated the first practice sessions on Friday, showing that Mercedes was still the team to beat this season, but Bottas and Massa were hot on their heels.
Except for Montreal two weeks ago, Mercedes have had a perfect season, taking all first six races, which has seen it build a 119-point lead in the constructors` standings ahead of Red Bull.
After an already average performance on Friday, the Austrian team continued to struggle Saturday, with Ricciardo coming in seventh place, and reigning four-time champion Sebastian Vettel in 10th.
Red Bull had hoped this weekend to capitalise on its recent win in Canada, especially with a race on home turf in Austria.
But its drivers admitted ahead of the final practice that they would not be going into Sunday`s race as the favourites, with Mercedes too strong at the moment.
Under an overcast sky that was nevertheless an improvement on Friday`s light rain and windy conditions, the teams started on soft tyres before going into the supersofts.
Several cars suffered minor problems during the session, with McLaren`s Jenson Button going far off at one point after the first turn and reporting brake problems. Button`s session ended with just four laps in the bag.
Qualifying will take place at 2:00pm (1200GMT).