Sebastian Vettel waited until his last lap of free practice in Valencia to reveal a glimpse of Red Bull's true pace ahead of qualifying for the European Grand Prix at Valencia.
The German's time of 1m38.052s, using the F-duct-equipped RB6 on the option super-soft Bridgestones, denied a rapid Robert Kubica from a moment of glory just as the chequered flag waved for the end of the hour.
The Pole had made full use of Renault's new blown-diffuser system to throw his hat into the ring as a potential poleman, late in the session, but his 1m38.154s lap was 0.102s short of Vettel's best in the end.
Both men's laps had come in an intense flurry of activity in the last 10 minutes of the session as the cars, drained of fuel, were attached to sets of option tyres and released on to the track for qualifying simulations.
But even before that, the top ten had been covered by less than second for much of the hour, indicating an unpredictably competitive weekend. It still remains unclear which team has made the most gains with their upgrades.
Fernando Alonso looked quickest of all when the field was running the prime tyre, fending off advances from the Red Bulls and Adrian Sutil's Force India. Having already confirmed the potential of Ferrari's updated F10 chassis on Friday, he set the pace through much of the hour on the prime tyre - working down to a 1m38.513s.
But in the last 10 minutes, the Spaniard drifted down to fifth place, behind third-placed Mark Webber and a consistently fast Adrian Sutil.
Rubens Barrichello was a surprise sixth fastest, having pulled one out of the hat in a last gasp effort for Williams, ahead of Vitantonio Liuzzi in the second Force India.
Felipe Massa was an unhappy eighth, as the Brazilian spent his hour clambering kerbs and catching the rear-end of his Ferrari.
The McLarens of Jenson Button and Lewis Hamilton completed the top ten - neither having troubled the fight at the front. Hamilton also spent some time in the pits dealing with a problem with the front left corner of his car.
Nico Rosberg was an early front-runner for Mercedes before dropping back to 11th ahead of Sebastien Buemi, Nico Hulkenberg and Vitaly Petrov. Michael Schumacher was a distant 15th.
Pos Driver Team Time Laps
1. Vettel Red Bull-Renault 1:38.052 14
2. Kubica Renault 1:38.154 + 0.102 17
3. Webber Red Bull-Renault 1:38.313 + 0.261 13
4. Sutil Force India-Mercedes 1:38.500 + 0.448 17
5. Alonso Ferrari 1:38.513 + 0.461 18
6. Barrichello Williams-Cosworth 1:38.623 + 0.571 15
7. Liuzzi Force India-Mercedes 1:38.676 + 0.624 17
8. Massa Ferrari 1:38.686 + 0.634 16
9. Button McLaren-Mercedes 1:38.769 + 0.717 16
10. Hamilton McLaren-Mercedes 1:38.816 + 0.764 15
11. Rosberg Mercedes 1:38.822 + 0.770 15
12. Buemi Toro Rosso-Ferrari 1:39.050 + 0.998 16
13. Hulkenberg Williams-Cosworth 1:39.105 + 1.053 15
14. Petrov Renault 1:39.113 + 1.061 16
15. Schumacher Mercedes 1:39.222 + 1.170 14
16. Alguersuari Toro Rosso-Ferrari 1:39.392 + 1.340 18
17. Kobayashi Sauber-Ferrari 1:39.527 + 1.475 16
18. de la Rosa Sauber-Ferrari 1:39.699 + 1.647 16
19. Kovalainen Lotus-Cosworth 1:41.303 + 3.251 19
20. Trulli Lotus-Cosworth 1:41.428 + 3.376 20
21. Glock Virgin-Cosworth 1:41.955 + 3.903 17
22. di Grassi Virgin-Cosworth 1:42.354 + 4.302 18
23. Senna HRT-Cosworth 1:42.611 + 4.559 18
24. Chandhok HRT-Cosworth 1:42.622 + 4.570 19
All Timing Unofficial