After storming from third to first with an audacious outside move past the Repsol Hondas into turn one, Ben Spies looked well on course for his fourth podium of the season at Aragon on Sunday.
The Texan, who had been the leading Yamaha rider for much of the weekend, soon settled into third behind Casey Stoner and Dani Pedrosa - setting his fastest lap of the race on lap four.
But a rapid deterioration in rear grip followed and a frustrated Spies had dropped back to fifth by the chequered flag.
“I knew the start wasn't going to last long the way Casey and Dani were riding but I felt pretty good in the beginning," he said. "Even when we got to the fourth or fifth lap I thought our advantage to fourth place was growing. I felt really comfortable and thought we had a podium fight in us.
"Then the sixth or seventh lap the tyre just made a huge drop and just kept going down. Usually they make one drop then stay consistent. I had no confidence in the rear and no grip. I'm frustrated as before that we were riding well and the bike was working great.”
Yamaha team director Massimo Meregalli felt the cooler race day temperatures had caused the rear tyre problems.
“The drop in temperature this afternoon probably compromised the work we've done up to this morning in set up. Ben really couldn't do more because the rear tyre was really bad after just a few laps," he said.