Bradley Smith, the first British rider to take a 125cc pole since Chas Mortimer in 1972, led the field through turn one - and was still in control when his Polaris World Aprilia struck difficulty during lap two of 18.
Smith was forced wide after his Aprilia shook its front viciously, the first sign of a mechanical problem that soon saw him dropping backwards through the field. Smith eventually nursed his sick machine home in a disappointing 16th - and he wasn't the only Aprilia rider to lose the lead due to a mechanical.
Reigning world champion Gabor Talmacsi inherited first position from Smith, while 15-year-old rookie Scott Redding - who became the youngest ever rider to qualify on the front row on Saturday - caught the top six to complete a lead group alongside Talmacsi, Sergio Gadea, Joan Olive, Raffaele de Rosa, Danny Webb and Mike di Meglio.
After several lead changes, Talmacsi was back up front by lap 15 of 18 - when his Aprilia suffered some form of technical failure, leaving the Hungarian to limp home in twelfth place. Further drama followed when de Rosa slammed into Olive during a failed overtaking attempt, sending de Rosa tumbling while Olive just hung on.
All of which meant that, with two laps to go, Gadea was leading from di Meglio, Redding, Olive and Stefan Bradl. Redding had been riding the wheels off his Blusens Aprilia - barging past Olive and slicing under Gadea earlier in the race - and was now in firm contention for a debut podium.
An injured Gadea was able to gain several vital tenths to stay just out of reach during the closing stages - handing the Spaniard only his second GP victory - while Olive recovered brilliantly to secure second position, but the fight for the final podium position went right to the line.
