Austrian Grand Prix winner Andrea Iannone led first MotoGP practice at Brno despite his first Ducati machine stopping on track.
Iannone claimed his maiden MotoGP victory last Sunday at the Red Bull Ring, also ending Ducati's run of nearly six years without a win.
The first session of the weekend's running for the Czech GP was largely dominated by Yamaha, with Jorge Lorenzo setting the pace in the first half with a 1m56.993s.
His team-mate Valentino Rossi then moved to the top with a 1m56.454s, but Lorenzo improved on two consecutive laps down to a 1m56.673s, 0.219 seconds behind.
Iannone had an inauspicious start to proceedings, his primary Ducati stopping on track with an apparent mechanical problem halfway through the session.
But with seven minutes to go he slotted in between the Yamaha pair, 0.200s off Rossi's pace, and then moved to the top entering the final minute with a time of 1m56.189s.
That lap was good enough to lead the way by 0.265s over Rossi and 0.484s over Lorenzo.
Avintia Ducati rider Hector Barbera jumped up to fourth on his final lap, just ahead of Iannone's team-mate Andrea Dovizioso.
Championship leader Marc Marquez had to settle for sixth on his Honda, 0.986s shy of Iannone.
Honda's Marquez was in a group of riders to have their best laptimes cancelled for exceeding track limits at the final corner, also including Scott Redding, who wound up eighth on his Pramac Ducati.
Suzuki's Maverick Vinales in seventh, Dani Pedrosa and Tech3 Yamaha rider Pol Espargaro in ninth and 10th also started their weekends inside the top 10.
Marc VDS Honda rider Jack Miller will miss the grand prix due to injuries sustained in his crash during last Sunday's Austrian GP warm-up.
|4||Hector Barbera||Avintia Ducati||0.775s||16|
|8||Scott Redding||Pramac Ducati||1.315s||18|
|11||Bradley Smith||Tech3 Yamaha||1.410s||17|
|12||Cal Crutchlow||LCR Honda||1.432s||16|
|13||Pol Espargaro||Tech3 Yamaha||1.897s||16|
|14||Danilo Petrucci||Pramac Ducati||2.132s||18|
|16||Yonny Hernandez||Aspar Ducati||2.471s||18|
|18||Tito Rabat||Marc VDS Honda||2.645s||21|
|19||Loris Baz||Avintia Ducati||2.927s||15|
|20||Eugene Laverty||Aspar Ducati||3.163s||16|