Dani Pedrosa grabbed the top spot at the end of the opening MotoGP free practice session at Aragon.
Pedrosa's Honda team-mate Casey Stoner had looked untouchable at the front for much of the morning as his early 1m50.806s lap put him half a second clear of the Spaniard and over a second ahead of everyone else.
But a flurry of late improvements saw Stoner reeled in and toppled. Jorge Lorenzo was first to beat the championship leader with a 1m50.7s lap on his Yamaha, before Pedrosa came through with first a 1m50.4s then a 1m50.281s to put himself nearly half a second in front of his rivals.
Lorenzo and Stoner remained second and third, with the world champion the only man in the top five not on a factory-spec Honda as Gresini's Marco Simoncelli and works team rider Andrea Dovizioso took fourth and fifth.
Randy de Puniet put in several rapid laps on the Pramac Ducati and was as high as third for a while before jumping back up to sixth having been shuffled down the order.
The works Ducatis were seventh and eighth. Nicky Hayden was 0.2s quicker than Valentino Rossi, who tried a revised version of the GP11 with a partially aluminium frame in the second half of the session. His best time - 1.395s down on Pedrosa - came on the latest specification bike.
Ben Spies was back in 11th on the second works Yamaha, behind Colin Edwards (Tech 3 Yamaha) and Hiroshi Aoyama (Gresini Honda), but had been in the top three before all the last-minute quick laps.
Pos Rider Team/Bike Time Gap 1. Dani Pedrosa Honda 1m50.281s 2. Jorge Lorenzo Yamaha 1m50.730s + 0.449s 3. Casey Stoner Honda 1m50.806s + 0.525s 4. Marco Simoncelli Gresini Honda 1m50.893s + 0.612s 5. Andrea Dovizioso Honda 1m51.091s + 0.810s 6. Randy de Puniet Pramac Ducati 1m51.376s + 1.095s 7. Nicky Hayden Ducati 1m51.418s + 1.137s 8. Valentino Rossi Ducati 1m51.676s + 1.395s 9. Colin Edwards Tech 3 Yamaha 1m51.826s + 1.545s 10. Hiroshi Aoyama Gresini Honda 1m51.859s + 1.578s 11. Ben Spies Yamaha 1m51.911s + 1.630s 12. Hector Barbera Aspar Ducati 1m52.016s + 1.735s 13. Alvaro Bautista Suzuki 1m52.224s + 1.943s 14. Cal Crutchlow Tech 3 Yamaha 1m52.580s + 2.299s 15. Karel Abraham Cardion Ducati 1m53.158s + 2.877s 16. Toni Elias LCR Honda 1m53.440s + 3.159s 17. Loris Capirossi Pramac Ducati 1m53.842s + 3.561s