Lorenzo quickest in opening practice
World Champion Jorge Lorenzo of Spain (L) takes a question during a press conference as compatriot Marc Marquez watches during the lead up to the Australian MotoGP Grand Prix at Phillip Island on October 17, 2013
Yamaha's Lorenzo clocked one minute 29.167 seconds for the fastest lap ahead of compatriot Marquez on a Honda by 0.088secs.
Marquez can claim the world title in his debut premier class season if he beats Lorenzo by sufficient points to carry a 50-point margin into the last two races in Japan and Spain.
The 20-year old Spanish star holds a 43-point lead heading into Sunday's Australian showdown and a win, worth 25 points, could land the title if Lorenzo cannot finish second.
If that happens Marquez would become the first rookie to clinch the premier class championship since American Kenny Roberts in 1978.
Spain's Alvaro Bautista had the third-fastest lap Friday, 0.644secs down on Lorenzo with Marquez's Spanish teammate Dani Pedrosa fourth at 0.841secs.