Fernando Alonso fired an ominous warning to the rest of the field as his qualifying simulation run saw him set the fastest time of the weekend at the end of the final free practice session for the Bahrain Grand Prix.
The Ferrari driver was the first to engage in a low fuel 'qualifying run' and set a 1m54.099s lap time.
But Nico Rosberg proved that qualifying will be a thriller by almost eclipsing the double world champion moments later, the Mercedes driver setting faster first and second sector times, only to lose out in the third. The German ended the session second, 0.269s back.
Mark Webber moved up to third for Red Bull ahead of Michael Schumacher, who was still trailing his team-mate Rosberg by a couple of tenths.
Sebastian Vettel was fifth fastest ahead of Felipe Massa whose final attempt was squandered as he ran wide in Turn 22. The lap was still good enough for sixth.
Jenson Button was seventh for McLaren, while Robert Kubica, Vitantonio Liuzzi and Nico Hulkenberg completed the top ten.
Red Bull's drivers were the first to get serious in the session as the team tried to make up for the time it perceived to have lost on Friday, and Sebastian Vettel was quickly to down a 1m55.178s - the fastest lap of the weekend so far.
He was joined by team-mate Webber, and then, within the next five minutes most of the rest of the frontrunners.
An interesting five minutes saw the top positions switch around a number of times as first Ferrari's Fernando Alonso, then team-mate Felipe Massa lowered Vettel's mark. The Brazilian, returning to competition following his head injury sustained in Hungary last year, set a 1m54.761s.
Behind him Rosberg all but matched it to go 0.084s and split the Ferraris.
While all this was going on there was drama for Virgin as Timo Glock turned into a tight left-hander, then watched in horror as his left-front wheel departed the car and rolled off on its own trajectory. The problem was apparently identified to an issue with an under-torqued airgun.
Others struggling during the morning were Glock's team-mate Lucas di Grassi, who had a gearbox problem, and Karun Chandhok, whose HRT Dallara failed to find any gear - rendering the Indian wihout any practice experience ahead of qualifying.
Perhaps more significantly for the outcome of the weekend however, Lewis Hamilton was forced to miss out on his qualifying simulation run at the end of the session after a problem with his McLaren's front left brake duct.
Pos Driver Team Time Laps 1. Alonso Ferrari 1:54.099 14 2. Rosberg Mercedes 1:54.368 + 0.269 13 3. Webber Red Bull-Renault 1:54.500 + 0.401 15 4. Schumacher Mercedes 1:54.533 + 0.434 13 5. Vettel Red Bull-Renault 1:54.646 + 0.547 16 6. Massa Ferrari 1:54.739 + 0.640 14 7. Button McLaren-Mercedes 1:55.000 + 0.901 14 8. Kubica Renault 1:55.331 + 1.232 15 9. Liuzzi Force India-Mercedes 1:55.432 + 1.333 11 10. Hulkenberg Williams-Cosworth 1:55.461 + 1.362 13 11. Sutil Force India-Mercedes 1:55.521 + 1.422 13 12. Hamilton McLaren-Mercedes 1:55.860 + 1.761 8 13. de la Rosa Sauber-Ferrari 1:56.063 + 1.964 14 14. Barrichello Williams-Cosworth 1:56.259 + 2.160 15 15. Buemi Toro Rosso-Ferrari 1:56.295 + 2.196 17 16. Alguersuari Toro Rosso-Ferrari 1:56.504 + 2.405 16 17. Kobayashi Sauber-Ferrari 1:56.530 + 2.431 13 18. Petrov Renault 1:56.811 + 2.712 16 19. Glock Virgin-Cosworth 1:59.173 + 5.074 10 20. Kovalainen Lotus-Cosworth 1:59.789 + 5.690 15 21. Trulli Lotus-Cosworth 2:01.259 + 7.160 5 22. Senna HRT-Cosworth 2:04.001 + 9.902 11 23. di Grassi Virgin-Cosworth 2 24. Chandhok HRT-Cosworth All Timing Unofficial