Nico Terol brought himself back into 125cc title contention by taking a comfortable third win of the year at Indianapolis, while Marc Marquez crashed while leading.
Marquez looked like he was striding towards his sixth win of 2010 and an extended championship lead, as he established a 2.6-second gap back to Terol, who had worked his way clear of the next pack.
But on lap nine Marquez lost control and crashed his Ajo Derbi - and although the bike was undamaged and he could rejoin, by the time he was upright again, he had fallen to 14th.
That left Terol (Aspar Aprilia) with a massive 9s lead, which he calmly allowed to dwindle to 4.9s by the flag, as Marquez's Ajo team-mate Sandro Cortese emerged in a lonely second.
Cortese had earlier been involved in a huge dice with Bradley Smith, until the Aspar rider crashed out, and title contender Pol Esparagro, who he eventually left behind. Efren Vazquez also fell away from the podium fight as he finished fourth.
Marquez charged back to fifth place in the end, but is unlikely to keep the position, as he made huge gains when he cut Turns 3 and 4 as he ran off-track following a mistake with two laps to go.
Esteve Rabat, Danny Webb and Randy Krummenacher are all set to benefit if Marquez is punished, as his short-cut set him up to pass all three in the final moments.
Luis Salom was ahead of this whole pack and on course for a career best fifth until going off and dropping to 12th late on.
Pos Rider Bike Time/Gap 1. Nicolas Terol Aprilia 42m19.223s 2. Sandro Cortese Derbi + 4.995s 3. Pol Espargaro Derbi + 10.856s 4. Efren Vazquez Derbi + 15.402s 5. Esteve Rabat Aprilia + 19.912s 6. Danny Webb Aprilia + 20.093s 7. Randy Krummenache Aprilia + 20.702s 8. Alberto Moncayo Aprilia + 26.797s 9. Jonas Folger Aprilia + 27.666s 10. Marc Marquez Derbi + 39.840s 11. Jasper Iwema Aprilia + 46.153s 12. Luis Salom Aprilia + 52.556s 13. Johann Zarco Aprilia + 54.354s 14. Jakub Kornfeil Aprilia + 54.402s 15. Lorenzo Savadori Aprilia + 1m07.208s 16. Marco Ravaioli Lambretta + 1m24.117s 17. Simone Grotzkyj Aprilia + 1m25.890s 18. Luca Marconi Aprilia + 1m53.618s Retirements: Tomoyoshi Koyama Aprilia 14 laps Bradley Smith Aprilia 12 laps Adrian Martin Aprilia 12 laps Zulfahmi Khairuddin Aprilia 11 laps Marcel Schrotter Honda 8 laps Isaac Vinales Lambretta 3 laps Louis Rossi Aprilia 1 lap