After leading the slippery first practice session at Indianapolis, reigning MotoGP champion Casey Stoner was knocked back to fifth by the end of the afternoon.

The Repsol Honda rider and defending Indy winner was left 0.840s from pace setting team-mate Dani Pedrosa after suffering issues with the rear of the bike, which are currently being investigated.

"We tried a couple of things this morning but with the track being so slippery we didn't really take anything from it," began Stoner.

"This afternoon we knew what we wanted to try after the morning session but unfortunately we had some issues with the bike.

"We had a problem with electronics on one exit and then a strange feeling in the rear on a later exit, perhaps a stone got into the chain or something, we're not sure yet.

"Due to the fact we couldn't identify what the problem was, we decided to use the second bike and I only managed a few laps.

"A slightly disappointing afternoon and we know what we want to work on tomorrow morning."