Reigning MotoGP world champion Casey Stoner began the first of his 15 remaining MotoGP events before retirement on top of the timesheets by over half-a-second at Le Mans on Friday morning.

The 26-year-old announced the shock news that he will leave at the end of this season during a press conference at the circuit on Thursday evening, but warned he would be pushing as hard as ever for the remainder of the season.

True to his word, the Repsol Honda star, who moved to the top of the 2012 standings after his second win of the season last time out at Estoril, led opening practice for the French Grand Prix by a decisive 0.587sec over team-mate Dani Pedrosa.

Stoner, whose best lap was his 15th and last, suffered a scare when his RCV 'switched off' for s few seconds on the exit of a turn, pushing the front.

Behind the Australian, one-second separated Pedrosa from tenth place Valentino Rossi (Ducati), with 2011 podium finisher Andrea Dovizioso starting strongly for Monster Yamaha Tech 3 in third place.

Factory M1 rider Ben Spies was an early fourth (+0.881s) with Hector Barbera putting in another fast practice lap to claim fifth place and top Ducati for the Pramac team.

Former Le Mans winner Lorenzo, who trails Stoner by just a single point in the championship, began the weekend in sixth, a fraction ahead of Dovizioso's team-mate Cal Crutchlow, who was the final rider within one-second of Stoner.

Alvaro Bautista claimed eighth for Honda Gresini, followed by factory Ductai team-mates Nicky Hayden and Rossi. Rossi took his only Ducati podium to date in last year's event.

Home star Randy de Puniet led the CRT challenge in 13th, behind the manufacturer machines, while Chris Vermeulen - making his MotoGP return in place of the injured Colin Edwards - was 21st and last for Forward Racing.

The Australian, winner at Le Mans in 2007, left grand prix at the end of 2009 and has not been able to test the Suter-BMW prior to this weekend's ride. Vermeulen was 7.077s slower than countryman Stoner.

Free practice (1):

1. Casey Stoner
2. Dani Pedrosa
3. Andrea Dovizioso
4. Ben Spies
5. Hector Barbera
6. Jorge Lorenzo
7. Cal Crutchlow
8. Alvaro Bautista
9. Nicky Hayden
10. Valentino Rossi
11. Karel Abraham
12. Stefan Bradl
13. Randy De Puniet
14. Aleix Espargaro
15. Mattia Pasini
16. Yonny Hernandez
17. Michele Pirro
18. James Ellison
19. Danilo Petrucci
20. Ivan Silva
21. Chris Vermeulen