(Reuters) - Triple world champion Jorge Lorenzo recognized he had work to do after lapping only 17th fastest for Ducati at MotoGP's first official pre-season test in Malaysia on Monday.

The surprised Spaniard, switching teams after nine years with Yamaha, said there was a big difference on the Sepang circuit between the bike he rode last year and the Italian machine.

"I think that to go a lot faster we have to do two things. The first is to adapt the bike bit by bit to how I like it, and the second and most important is to understand how to go fast," Lorenzo said.

"To get there I’m missing quite a lot, more than I expected, honestly, but it’s still very early to draw conclusions. It’s a question of putting in the hours and the laps to bit by bit find how to go fast."

Lorenzo, who also tested with Ducati in Valencia in November, was reassured at least that the bike had good potential.

Ducati test rider and retired double world champion Casey Stoner was fastest in one minute 59.681 seconds before rain cut short the session with two hours to go.

Lorenzo's Italian team mate Andrea Dovizioso, winner in Malaysia last year, was second fastest with Yamaha's new signing Maverick Vinales third. The Ducati riders were the only ones to lap inside two minutes.

World champion Marc Marquez was ninth fastest for Honda.

"The feeling today wasn’t so bad considering that the first day after the holiday break is always a bit demanding for the rider and the team," the champion said.

The season starts in Qatar on March 26.

(Reporting by Alan Baldwin, editing by Ed Osmond)