Monster Yamaha's Johann Zarco had a return to form in the opening MotoGP free practice at Brno, pinching top spot from Andrea Dovizioso by less than a tenth of a second with his final lap.
Dovizioso had taken charge of the timesheets with half an hour left to run, sticking in a 1'56.720 using medium Michelins all round to take him a tenth of a second clear of reigning champion Marc Marquez.
It looked like the Italian was set to stay there but Zarco had other ideas, putting in a 1'56.647 on a blisteringly-hot track to finish as top Yamaha also on mediums all round.
Valentino Rossi, on the works M1, ended in fifth place and under a quarter of second off the pace but it was Alvaro Bautista who provided the biggest shock, putting in fast laps with soft and medium tyres, ending in fourth on a 1'56.831.
Andrea Iannone, who has a tweaked chassis and new electronic settings this weekend, took sixth with a 1'57.079 with Dani Pedrosa next along.
Wildcard Stefan Bradl, who is running under HRC colours, ended in eighth ahead of Jack Miller and Tito Rabat. Cal Crutchlow is just outside the top ten in 11th with Jorge Lorenzo in 12th place.
Rookie Hafizh Syahrin put his satellite Monster Yamaha in 13th and one ahead of Maverick Vinales in 14th with Danilo Petrucci completing the top 15.
Brno is not a good venue for Aprilia as the mutliple hard acceleration points don't play to its strengths. Scott Redding ended down in 23rd with the recovering Aleix Espargaro one place further back.