Heikki Kovalainen snatched the fastest time from Nico Rosberg in the closing seconds of the Friday morning session at the Hungaroring.
The McLaren driver jumped up the order with his final lap and managed to pip Rosberg's Williams by 0.059 seconds.
Emphasising McLaren's strong form, Lewis Hamilton was third fastest, ahead of Mark Webber, who had led the way for much of the morning and was in front until the final scramble for quick laps.
Webber (Red Bull) had deposed Robert Kubica and Felipe Massa at the top just after the halfway point as the major contenders finally started their first serious runs of the day following a quiet start.
The Nurburgring winner's 1m22.615s lap was sufficient to keep him in front until the final six minutes, when first Hamilton, then Rosberg went slightly quicker.
Hamilton was on course to retake the top spot just after Rosberg had deposed him, but abandoned the lap following a small error in the middle sector. Rosberg then looked set to extend his advantage, before he too lost time near the end of the lap.
That meant Kovalainen was able to grab first place away with a last minute surge that saw the McLaren vault from the lower end of the top ten to the head of the order.
Kazuki Nakajima underlined Williams' strong session by taking fifth place, ahead of Toyota's Jarno Trulli, the two Ferraris and Fernando Alonso (Renault).
It was another low-key session for Brawn GP, with points leader Jenson Button ending up only 10th, 0.852s off Kovalainen's pace.
Debutant Jaime Alguersuari made a very respectable start to his Formula 1 career. The Spanish novice and his Toro Rosso team-mate Sebastien Buemi had the track to themselves for much of the first 40 minutes as others waited for the surface to clean, and by the end of the session there was just 0.230s between the duo in 19th and 20th places.
A stall at the end of the pitlane while practising a start sequence was the only blemish on Alguersuari's morning.
Pos Driver Team Time Laps
1. Kovalainen McLaren-Mercedes (B) 1:22.278 21
2. Rosberg Williams-Toyota (B) 1:22.337 + 0.059 27
3. Hamilton McLaren-Mercedes (B) 1:22.554 + 0.276 20
4. Webber Red Bull-Renault (B) 1:22.615 + 0.337 21
5. Nakajima Williams-Toyota (B) 1:22.619 + 0.341 25
6. Trulli Toyota (B) 1:22.705 + 0.427 17
7. Raikkonen Ferrari (B) 1:22.796 + 0.518 23
8. Massa Ferrari (B) 1:22.855 + 0.577 25
9. Alonso Renault (B) 1:23.001 + 0.723 28
10. Button Brawn-Mercedes (B) 1:23.130 + 0.852 25
11. Kubica BMW-Sauber (B) 1:23.146 + 0.868 24
12. Heidfeld BMW-Sauber (B) 1:23.154 + 0.876 23
13. Barrichello Brawn-Mercedes (B) 1:23.209 + 0.931 24
14. Glock Toyota (B) 1:23.234 + 0.956 30
15. Vettel Red Bull-Renault (B) 1:23.283 + 1.005 25
16. Fisichella Force India-Mercedes (B) 1:23.484 + 1.206 20
17. Piquet Renault (B) 1:23.678 + 1.400 22
18. Sutil Force India-Mercedes (B) 1:23.845 + 1.567 18
19. Buemi Toro Rosso-Ferrari (B) 1:23.998 + 1.720 37
20. Alguersuari Toro Rosso-Ferrari (B) 1:24.228 + 1.950 42
All Timing Unofficial