Sebastian Vettel grabbed the fastest time for Red Bull in a frantic end to final practice in Turkey.
It looked like McLaren's Lewis Hamilton would end the morning on top - despite a wild mid-session spin - as the Red Bulls both slipped up on their last runs.
But after exploring the Turn 8 run-off on his first flying lap at the end, Vettel regrouped and pulled a lap together on his next tour and managed to set a 1m27.086s - putting him nearly 0.3 seconds clear of the field.
Nico Rosberg jumped to second for Mercedes moments later, pushing Hamilton back to third.
Mark Webber was fourth at the end of an eventful morning that started when his Red Bull slowed to a crawl due to a throttle problem soon after leaving the pits for his first proper run. Having had an agonisingly slow lap of the track, he eventually made it back to the garage.
Just 12 minutes later, Webber rejoined in a healthy car and took no time at all to get up to second behind team-mate Vettel, who was quickest for most of the morning.
When they switched to soft tyres, Webber had trips over the run-off at Turns 1 and 4 before finally having a spectacular spin at Turn 8 on his last lap. He did not hit anything in any of the excursions and still ended up fourth.
Hamilton's mid-session problem was equally spectacular - as he ran wide halfway through Turn 8 and snapped into a spin. He held the McLaren in a high-speed sideways slide until it reached one of the very few patches of gravel trap at the Istanbul Park track, in which he was beached for a while before being extricated and allowed to return to the pits. The slide had shredded both rear tyres, but the McLaren eventually made it back with apparently little damage.
Robert Kubica was competitive all morning and took fifth for Renault, ahead of Fernando Alonso in the faster of the Ferraris, Mercedes' Michael Schumacher and Jenson Button (McLaren).
The most costly issue of the session was the hydraulic glitch that struck Adrian Sutil's Force India. The German's car was wheeled back into the garage without completing a flying lap and could not be repaired before the end of the hour.
Pos Driver Team Time Laps 1. Vettel Red Bull-Renault 1:27.086 18 2. Rosberg Mercedes 1:27.359 + 0.273 16 3. Hamilton McLaren-Mercedes 1:27.396 + 0.310 14 4. Webber Red Bull-Renault 1:27.553 + 0.467 15 5. Kubica Renault 1:27.784 + 0.698 20 6. Alonso Ferrari 1:27.861 + 0.775 18 7. Schumacher Mercedes 1:27.879 + 0.793 16 8. Button McLaren-Mercedes 1:27.963 + 0.877 17 9. Massa Ferrari 1:27.969 + 0.883 20 10. Petrov Renault 1:28.344 + 1.258 18 11. Buemi Toro Rosso-Ferrari 1:28.610 + 1.524 22 12. de la Rosa Sauber-Ferrari 1:28.652 + 1.566 20 13. Alguersuari Toro Rosso-Ferrari 1:28.734 + 1.648 21 14. Kobayashi Sauber-Ferrari 1:29.036 + 1.950 20 15. Hulkenberg Williams-Cosworth 1:29.044 + 1.958 18 16. Liuzzi Force India-Mercedes 1:29.211 + 2.125 15 17. Barrichello Williams-Cosworth 1:29.305 + 2.219 14 18. Trulli Lotus-Cosworth 1:30.618 + 3.532 19 19. Kovalainen Lotus-Cosworth 1:30.884 + 3.798 22 20. Glock Virgin-Cosworth 1:31.341 + 4.255 21 21. di Grassi Virgin-Cosworth 1:32.180 + 5.094 16 22. Senna HRT-Cosworth 1:32.230 + 5.144 21 23. Chandhok HRT-Cosworth 1:32.762 + 5.676 19 24. Sutil Force India-Mercedes No time 1 All Timing Unofficial