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Yonny Hernandez led the way during a wet, incident packed Free Practice 1 for the San Marino MotoGP at Misano.
Heavy rain and cool temperatures created treacherous conditions that caught out eight riders - Bradley Smith, Aleix Espargaro, Hiroshi Aoyama, Karel Abraham, Andrea Iannone, Pol Espargaro, Valentino Rossi and Cal Crutchlow - in the opening 15 minutes.
Most lost the front, even when almost upright under braking, as riders struggled with both the weather and generating enough heat in the harder Bridgestone wet tyres.
All escaped serious injury although Iannone limped away after a knock to his right leg and was later seen with an ice pack on his foot. Pol Espargaro had fallen seconds after Iannone into turn one, their bikes colliding in the gravel.
Unsurprisingly the incidents prompted many riders to remain in the pits, especially with dry weather predicted for Saturday and Sunday.
World champion Marc Marquez was leading the timesheets as the non-stop falls came to an end, with former Supermoto star Yonny Hernandez taking over for the home Pramac team at the halfway stage.
The Colombian remained on top to the end of the 45-minutes, finishing 1.693s clear of Marquez's Repsol Honda with Andrea Dovizioso third for home manufacturer Ducati. Stefan Bradl was next best for LCR Honda as the top four completed less than eight laps.
Danilo Petrucci put in the most laps (12) on his way to fifth place, but also became the ninth and final faller of the session courtesy of a highside from his Ioda ART in the closing minutes.
By contrast Smith and PBM's Broc Parkes were not credited with a fast lap.
Free Practice 2 takes place this afternoon and is also forecast to be wet.
Free Practice 1:
1. Yonny Hernandez
2. Marc Marquez
3. Andrea Dovizioso
4. Stefan Bradl
5. Danilo Petrucci
6. Aleix Espargaro
7. Andrea Iannone
8. Pol Espargaro
9. Valentino Rossi
10. Dani Pedrosa
11. Alex De Angelis
12. Jorge Lorenzo
13. Cal Crutchlow
14. Karel Abraham
15. Michael Laverty
16. Scott Redding
17. Alvaro Bautista
18. Hiroshi Aoyama
19. Mike Di Meglio
20. Leon Camier
21. Hector Barbera