The rain was torrential for long periods, meaning very little track action for much of the day.
Only when conditions eased slightly early in the afternoon did the field take to the circuit in earnest.
Lorenzo managed to set a 1m47.423s on his Yamaha, putting him one second clear of Honda duo Dani Pedrosa and Marc Marquez.
Ducati had a happier time in the wet than it has in the dry of late, filling the final top five spots with Pramac's Andrea Iannone and factory test rider Michele Pirro.
Valentino Rossi was back in sixth on the second Yamaha.
Paul Bird rider Yonny Hernandez led the CRT pack in 12th overall.
Testing continues through Sunday and Monday.
- Sports & Recreation
- Dani Pedrosa