While Fenati ran at the front throughout, set a flurry of fastest laps at the end and lapped several riders on the way to a 30-second victory that also gave him the points lead, his win also owed to the high number of riders who fell foul of the circuit conditions - with just 17 riders able to finish the race.
The tone of the race was set from the start, as three riders - including Jack Miller from second - all high-sided at the damp Turn 9.
Two laps later and Jakub Kornfeil, who was leading, lost the rear of his Ongetta FTR-Honda on the final corner. He managed remarkably to hang on to the bike and stay upright, only to drop the machine on the wet astroturf as he attempted to rejoin.
That left Alex Rins, Fenati and Louis Rossi in a three-way fight for the lead, and it was Rins who looked the most likely of the trio as he began to stretch away.
His advantage was undone when he too slipped off track and was passed by both Fenati and Rossi. The order lasted all of one lap, as Rossi then dropped his FTR-Honda at Turn 1 and surrendered what would have been a career-best finish.
Fenati then began to pull away from Rins, before the Spaniard dropped it at Turn 1. Once again he was able to get going, but his fight for victory was over as he slipped back into the chasing pack of Sandro Cortese, Luis Salom and Alexis Masbou.
The quartet traded places almost corner by corner in the final laps their battle a direct contrast to the serenity of Fenati, 34s down the road with Salom eventually claiming second ahead of Cortese and Rins.
Qatar race winner Maverick Vinales was another early victim, running through the gravel on lap three, but the championship favourite staged a remarkable recovery and fought his way back to sixth at the finish.
Alberto Moncayo, Niccolo Antonelli and Hector Faubel rounded out the top nine, having been locked in a fight for sixth throughout the race.