MOTEGI, Japan (AFP) - World champion Jorge Lorenzo lapped fastest in practice on a crash-filled Friday ahead of this weekend's Japanese MotoGP Grand Prix, as Dani Pedrosa fractured his collar-bone in a nasty spill.

Championship leader Marc Marquez was fourth quickest for Honda but much of the focus was on Pedrosa, who decided to fly back to Spain for surgery after a heavy fall in Friday's afternoon session that broke his collar-bone into four pieces.

Yamaha's Lorenzo clocked 1min 45.151sec in cool and dry conditions at Motegi, followed by Ducati's Andrea Dovizioso, who lapped 0.053sec slower, with rising Suzuki star Maverick Vinales third.

"The bike is working quite well and I'm in good shape, so I'm very motivated to make it a good race," Lorenzo said.

Marquez suffered a minor fall in the afternoon as he tested the limit of the hard front tyre at the hairpin but was still good enough for fourth, while his closest championship rival Valentino Rossi was seventh.

"We are very close," said Spain's double world champion. "At the moment Lorenzo is the fastest guy. But me, Vinales, Valentino, Dovizioso - we more or less have the same pace."

The hard tyre also caused troubles for Rossi, as temperature quickly dropped in the afternoon in mountainous Motegi, sending several riders to crash.

"It was a mixed day. This afternoon it was difficult practice because we tried the hard tyre," said the Italian.

Along with Pedrosa, Aspar's Eugene Laverty also wiped out at turn six.

Both men managed to walk to the circuit's medical centre. Laverty decided to seek further tests at a local hospital, while Honda decided to send Pedrosa home. He will be replaced by test rider Hiroshi Aoyama, team officials said.

"I was on the out-lap of my last run in (afternoon practice) when I momentarily lost the rear entering turn 11, and when the tyre found grip again it launched me in the air," Pedrosa, who is set to undergo surgery in Barcelona on Saturday, said in a statement.

Marquez, meanwhile, can mathematically claim his third MotoGP title in Japan on Sunday.

With four races to go, Rossi is battling to close a 52-point gap against Marquez, who has never won at the Motegi circuit but his second-place finish in 2014 secured that season's championship.

Sunday's race kicks off three weekends of successive races in an Asian "triple-header", which also takes the riders to Australia and Malaysia, before the season finale in Spain after a topsy-turvy season that has yielded eight different grand prix winners so far.