World championship leader Marc Marquez was gifted his fourth consecutive 125cc win of the season after his key title rival Pol Espargaro crashed out of the lead with three laps to go.
The pair had battled tooth-and-nail at the front from lap four, when they passed early (wet tire-shod) early leaders Jonas Folger and Marcel Schrotter when the track was still mostly damp. Swapping the lead virtually every lap from that point onwards, and pushing hard as they explored the limits of the rumble strip's adhesion at several points around the circuit, it remained unclear which rider ultimately held a pace advantage heading in to the final stages.
Though it seemed as if Marquez's was the quicker Derbi in the final third of the lap and Espargaro always looked slightly more on the edge at that point on the circuit. So it proved when Espargaro went an inch too wide on the fast penultimate right-hander and ran on to the Astroturf. He immediately lost purchase and flew high in the air, releasing Marquez in to a clear 23sec lead.
Espargaro's mistake elevated Tomoyashi Koyama to second, though he had to fight hard for that too. In a scrap that included Esteve Rabat, Bradley Smith, Randy Krummenacher and latterly the home-grown talent of Sandro Cortese, Koyama moved to the front just at the right time to inherit second.
Cortese who dropped back badly off the front-row at the start, applied determination and patience to fight his way back on to the tail of this battle for third - which became second after Espergaro's crash. And as it heated up in the final laps, Cortese kept his cool and scrapped his way through, and as they came into the final corner, made a wild dive down the inside of Rabat for third.
If the Spaniard hadn't sat up slightly, they both would have fallen, but he did and Cortese was through for a podium finish in front his home crowd. Rabat gathered it up for fourth ahead of Smith, who faded in the final laps having run third himself three laps from the flag.
Johann Zarco had a grandstand view of all this as he ran around just behind this pack to finish sixth. Danny Webb was another last-lap opportunist as he took seventh from Efren Vazquez.
Sturla Fagerhaug and wild card entry Daniel Kartheininger completed the top 10, while early leader Folger, who was at one point five seconds ahead of the winner in those opening wet laps eventually finished 16th.