Defending MotoGP World champion Jorge Lorenzo was lacking confidence with his Yamaha M1 as practice commenced at Sepang on Friday.

Lorenzo, who is 39 points behind Repsol Honda's Marc Marquez with four races remaining, finished the free practice sessions in fourth place behind team-mate Valentino Rossi after battling an 'aggressive' bike and mid-corner handling issues.

The Mallorcan is entering a crucial phase in the championship as he bids to challenge Marquez, with Sepang hosting the first of three consecutive flyaway rounds ahead of the finale at Valencia in November.

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"We are investigating what happened today because I didn't have confidence in the middle of the corners and also with acceleration," said Lorenzo.

"I had a lot of spinning and the bike is very aggressive so we have to see what we can change for tomorrow to try to get back a good feeling with the bike again."

Lorenzo set his best time of 2m 01.623s in FP1, leaving him fifth overall on the combined times after FP2. Lorenzo was 1.069s behind Dani Pedrosa, who led a Repsol one-two ahead of Marquez.

"We cannot control the performance of the other riders and all we can do is improve ourselves and it is not so good at the moment," said Lorenzo.

"To compare our performance to the Honda we lose most of our performance under braking. We can't improve the braking but if we can improve on acceleration and corning than we might win races.

"Obviously compared to the Honda it is very difficult to fight with them under braking.

"I have to positive and be ready and try to improve if you are not happy it is the start of the finish."