Reigning champion Marc Marquez of Spain, on a Honda, came second, just 0.014sec behind Lorenzo after topping the afternoon practice.
Italy’s Andrea Iannone, on a Ducati, took the third spot, trailing Lorenzo by 0.251sec.
Marquez’s team-mate and compatriot Dani Pedrosa, the 2014 Brno winner, came fourth ahead of the Italian duo of Andrea Dovizioso on a Ducati and championship leader Valentino Rossi on a Yamaha.
The nine-time world champion across all categories, Rossi leads the overall standings with 195 points after 10 out of the season’s 18 MotoGP races, ahead of Lorenzo with 186 and Marquez, the 2013 and 2014 overall champion, with 139.
Among the top six riders on Friday, only Marquez and Iannone managed to improve in the afternoon session.
With the tarmac heated up to 50 degrees Celsius (122 Fahrenheit), Marquez ran off the track in the morning practice and crashed at the end of the afternoon session.
Rossi also crashed in the afternoon but resumed the practice, while Pedrosa was less fortunate, walking straight to the mobile clinic with ice on his left foot following a crash early in the afternoon session.
“Dani Pedrosa is OK!” the clinic later said on Twitter.
Dovizioso, Bradley Smith and Mike Di Meglio also crashed at the end of the second practice.
Defending Moto2 champion Tito Rabat dominated the practice in his category while Moto3 championship leader Danny Kent of Britain topped the Moto3 practice standings. – Agence France-Presse