Casey Stoner emerged quickest after a frantic free practice session for the German Grand Prix at the Sachsenring, split equally between wet and dry conditions.

A torrential rain storm, which washed out the previous 125cc session, had blown over by the time the premier-class hour began, but it took until the start of the second half of the session before slick tyres could be used on the drying surface.

The change had an instant effect, dropping lap times from a best of 1min 32sec on wets to 1min 24sec within ten minutes of the switch to the softer compound slicks, denoted by a white line.

The constant stream of improvements at the front - by the likes of Nicky Hayden, Andrea Dovizioso, Dani Pedrosa, Stoner, Randy de Puniet and Valentino Rossi - finally came to a halt when a 1min 22.779sec set by Stoner with eight minutes to go couldn't be beaten.

Stoner, diagnosed with 'slight gastritis and mild anaemia' (according to the Ducati Marlboro Team) after a series of medical tests following Laguna Seca, starts this weekend's ninth round 16 points behind world championship leader Valentino Rossi.

Surprise US GP winner Dani Pedrosa, who lost the race and world championship lead when he crashed in last year's wet Sachsenring race, got closest to Stoner in free practice with a 1min 23.034sec.

Pedrosa, like Repsol Honda team-mate Dovizioso (ninth fastest), was due to try a new engine during the session, featuring a more user-friendly power delivery.

Rossi completed a top three covered by 0.309sec for Fiat Yamaha, with de Puniet the top satellite rider in fourth for LCR Honda, just ahead of fellow customer RC212V rider Alex de Angelis (San Carlo Honda Gresini). de Angelis scored a joint career-best fourth at Sachsenring last season.

Colin Edwards began his weekend with sixth for Monster Yamaha Tech 3, with Toni Elias seventh and Hayden eighth. The American, who claimed a Ducati-best fifth last time out in his home race, led most of the wet part of the session and continued that form after the change to slicks, before dropping back in the closing stages.

Hayate's Marco Melandri also led the timesheets in the wet before being ranked tenth in the dry.

Second in the championship Jorge Lorenzo, riding with foot and shoulder injuries sustained during a huge US GP qualifying crash, was only eleventh quickest - but just 0.717sec behind Stoner - with Loris Capirossi the lead Suzuki in twelfth.

Capirossi's team-mate Chris Vermeulen limped away from a big highside near the end of the session. The accident occurred when the Australian rode through a stream of water on the exit of turn one, causing a big rear wheel slide that finally pitched him into the air. Vermeulen finished 16th.

MotoGP rookie Mika Kallio, returning after a one-race absence caused by a finger injury at Assen, was 13th fastest and the first rider over one-second behind Stoner. As well as Vermeulen, the Pramac Ducati pilot was ahead of Tech 3's James Toseland, his team-mate Niccolo Canepa and Scot Honda's Gabor Talmacsi.

The MotoGP grid has been reduced to 17 permanent riders from Germany onwards, following the departure of the Grupo Francisco Hernando team, which had been running Sete Gibernau.

Mixed weather is expected to continue throughout the weekend.

Friday practice:

1. Stoner
2. Pedrosa
3. Rossi
4. de Puniet
5. de Angelis
6. Edwards
7. Elias
8. Hayden
9. Dovizioso
10. Melandri
11. Lorenzo
12. Capirossi
13. Kallio
14. Toseland
15. Canepa
16. Vermeulen
17. Talmacsi