Championship leader Marc Marquez topped the Czech Republic MotoGP free practice on Friday with the afternoon session disrupted by heavy rain that affected times.
Honda's reigning world champion took 1min 56.803sec to cover the 5.4km circuit in dry conditions on Friday morning.
Fellow Spaniard Jorge Lorenzo on a Yamaha was 0.037sec behind Marquez and 0.015sec ahead of Marquez's Honda teammate Dani Pedrosa.
Marquez set the best wet time ahead of Pedrosa and Italy's Valentino Rossi on a Yamaha.
Forecasters predict more rain for Saturday's practice and qualifying sessions.
On Sunday, the 21-year-old Marquez will be aiming for an 11th straight win in as many races this season, which would be a MotoGP record.
Last weekend at Indianapolis Marquez became the first rider since Australian Mick Doohan in 1997 to win 10 races in a row.
Italian legend Giacomo Agostini won all ten races of the premier class in 1968 and the first ten in 1969 for a streak of 20.
After 10 of 18 MotoGP races this year, Marquez leads the championship with a perfect 250 points, 89 ahead of fellow Catalan Pedrosa and 93 ahead of Rossi.
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