Stoner on top at Sepang

The Australian missed out on the first day of testing after injuring his back while doing a warm-up exercise, but he returned to action on Wednesday.

And although he had taken painkillers, Stoner set a fastest time of 2 minutes 0.895 seconds, finishing the session 0.157s ahead of Yamaha rider Ben Spies.

Jorge Lorenzo, who was fastest on the opening day of practice, was a little further behind his American team-mate in third.

Stoner's Repsol Honda team-mate Dani Pedrosa was over half-a-second behind the MotoGP champion in fourth, while the top five was rounded out by Britain's Cal Crutchlow.

Pain

The pain in Stoner's back is a lasting effect from a 2003 crash, but the 26-year-old says that it does not hinder his riding most of the time.

"There is nothing we have been able to do to stop it from happening. It just happens. So it is frustrating, I can't do anything about it at the moment," Stoner said.

He added: "Yesterday, it was far beyond anything I ever had before and I was worried. I will have more tablets tomorrow. So it should be OK."

Stoner did say that he enjoyed riding his new 1,000cc spec bike, claiming that they are more stable despite the increase in power.

"Definitely more power is a little bit more fun. You can square the corners off a little bit more. I actually find more traction (with the 1,000cc bike). I find this bike a little more stable," he said.

The first of MotoGP's pre-season tests concludes on Thursday.