Casey Stoner fastest in Sepang

MotoGP world champion Casey Stoner has shrugged off a sore back to post the fastest time in pre-season tests on the sport's new 1000cc bikes at Malaysia's Sepang circuit.

Stoner has described his recurring problem with back pain as 'frustrating' but is confident it won't hamper his world title

defence this season.

The Australian Honda rider, who won the championship in 2007 and again last year has been bothered by pain stemming from a 2003 crash and was sidelined on Tuesday while stretching his back before the first day of testing.

'There is nothing we have been able to do to stop it from happening,' he told reporters on Wednesday.

'It just happens. So it is frustrating ... I can't do anything about it at the moment,' he said.

Stoner said the pain mostly does not affect his riding but admitted that he took to the track on Wednesday after taking pain pills.

'It is just aspirins and painkillers,' he said.

Stoner said the back pain hits him at least three times a year, usually in cold weather. It generally occurs when he is not riding a bike and the pain is normally 'manageable'.

'Yesterday, it was far beyond anything I ever had before and I was worried,' he said.

'I will have more tablets tomorrow. So it should be OK.'

Medical scans have so far been inconclusive, he said, but added he would soon undergo new checks on his back and a stiff neck problem.

Stoner bounced back on Wednesday to outpace his Yamaha rivals in conditions that, while hot and humid, were drier than on day one and led to faster lap times as teams appeared to get the feel for the new bikes.

Stoner, who won 10 races last season, hit the top of the timesheet in late practice runs, logging 2 min 00.895 seconds while his Honda teammate Dani Pedrosa of Spain was fourth with a time of 2:01.508.

'When I went out the track was good. The bike was performing pretty well. It was nice to feel the wind on your face again,' Stoner told reporters.

MotoGP has moved to 1000cc bikes this year in place of the earlier 800cc engines.

American Yamaha rider Ben Spies, who was fastest in early practice on Wednesday, ended up in second place on 2:01.052 while his teammate Jorge Lorenzo of Spain, the 2010 world champion, was third with a best time of 2:01.068.

Yamaha's British rider Cal Crutchlow rounded out the top five with a time of 2:01.565.

'Yamaha (is) looking strong and competitive,' Stoner said.

'Jorge is doing a great job. Ben is starting the season strong. Hopefully they can continue that.'

The testing session marked an emotional return for riders to the Sepang circuit for the first time since Honda rider Marco Simoncelli died there in a shocking crash in October's Malaysian MotoGP.

Seven-time world champion Valentino Rossi of Ducati, who is looking to make up for a disappointing 2011 campaign, ended in seventh place on 2:01.886.

The 32-year-old Italian last year failed to win a race for the first time in his 15-year career.

Testing ends on Thursday and is followed by another three-day session at Sepang beginning on February 28.

The MotoGP 2012 season will officially start on April 8 in Qatar.